Framing gun is misfiring, shoots a nail about every 5 shots. My compressor reads 100 pounds air pressure. Internal piston is clean of debris.

A nailer will misfire if the next nail cannot advance to proper position. Make sure that the nails move freely in the rails. If the nails move freely then most likely the piston driver is not moving back up all the way to top of cylinder. This can happen if the piston oring is too loose or too tight. The piston oring should hold the piston in place without dropping down the cylinder. Also not so tight that there is too much friction. If the piston seems correct then check for debris in the head causing restriction of the discharge air. Remove the head valve to make sure all parts are ok. If headvalve and discharge port seems ok then air must be leaking past the nose of the gun. Under the bumper is a metal washer that helps to prevent too much air from leaking down the nose. You can make a good seal out of a piece of innertube to create more pressure for pushing the piston up. Cut a circle of rubber to fit under the bumper and on top of the metal washer, the bumper will hold it in place. Cut a slit or hole so that the piston driver will just fit snuggly. You should have solved the problem. Good luck with your repair.

If your using refills that have wire holding the nails together.
Check to see if bits of wire, debris is collecting in around the striking hammer. I switched to paper and never looked back. Must have gone through 5k worth of nails and rarely jams. Other things to check,
1. Check the striking hammer, make sure it is not bent or damaged.
2. Check air pressure, Low pressure can cause jamming, misfires.
3. If the tool needs pneumatic oil, 2-3 drops before you start good rule of thumb.
4. Check spring return. Sufficient pressure must be maintained or the nails will not feed properly. Misfires, incorrect feed, jamming etc.
I had a Hitachi brad nailer and it was horrible. Fire 1 brad nail would jam or misfire. Did everything I could think of nothing worked. Put it up, got a Rigid brad nailer and never looked at the Hitachi again. :)
Sometimes you just get a lemon! (PS I understand you have a framing nail gun. Brad nail gun above is used for an example)
Hope this helps

Your gun should work in the 90-120 psi range to get a consistant drive depth and with a few drops, 5-6 or a small squirt, of oil at the start of work and a few more in the afternoon if you are using it a lot. It is possible to use too much oil, if you over oil it and the bottom of the cylinder chamber gets full of oil or other gunk attracted to the oil it can get high enough to interfear with the return function. Also if you are using something other than pneumatic nail gun oil it is possible some o-rings were damaged. Either way you have to remove the head cap and check the internal o-rings for damage and clean out anything in the cylinder and head chamber.

This is usually low air pressure. The framing nailers need to have a minimum of about 90 pounds to work properly. I always run mine at 100. When using a too small compressor (such as the "pancake" compressors) and continuous firing they will do this after about six or seven shots. Once the compressor catches up they work fine again. Also make sure you put six or eight drops of gun oil in the air inlet each morning.

There is nothing on a nailer to adjust the air pressure. You have to do it at the pressure regulator. It is feasable to mount a regulator at the gun but more likely it's on the compressor. Most nail guns are rated for 90 to 120 psi. Set your regulator to hold the hose pressure between 90 and 100 psi. Using higher pressure to set the nail will ruin your nailer. Use the depth adjustment on the gun to set how far the nail is driven.

You did not described if there is piston movement when you squeeze the trigger or push outlet into the material. If there is piston movement but it is too weak to push the nail, insufficient air supply is first obvious reason.

Eliminate insufficient air pressure or volume supplied to the piece. Air pressure supplied should be between 70psi to 120psi (4.9bar to 8.3bar), air hose should be at least 7mm internal diameter.

If air supply was eliminated, disconnect air supply and check if piston is not locket in one position, you described that you done some maintenance on the machine. I assume therefore that you will know how to check piston for movement, if there is some restrain, you need to find what is causing it and remove obstruction.

As you put gun together, do not forget to lightly oil all assembly surfaces with compressor oil.

This is very good gun and should serve you for many years to come, we use these for last about 8 years rather haevily without any problem (except aprentices...), hope you will have as good service from yours.

Please send note what was the reason, and how did you solved it.

Could have the sequential trip trigger on the gun, there is a single shot trigger available! I believe the single shot is Grey in color!

the nail return spring for the piston is stuck or broken,dis mantle the gun and clean and check the spring to see if its broken

has the gun been cleaned and oiled, i think not.

take the head bolts out of the top of the gun. make sure you pay attention to how everything comes out so you can put it back together.

shake the gun upside down so that the piston comes out. wipe everything down with any petroleum based cleaning solvent to remove all the gunk and reassemble.

pour about 10 dropsof oil down the air input, let it sit there about 5 mins and hook up the air and it should work

Your air pressure is dropping everytime you fire. It sound like your compressor is not cutting back in untill it to low. Not giving your tool enough continuess pressure. Your compressor cut in pressure should be around 90-100psi no less. and your max. pressure no more then 120psi. (Make sure your regulator is set at 110psi)
good luck...!