What angle (25, 35, etc) collated nails should i purchase for this tool

First, check that you are using the proper nails. The FP602 needs Full Round Head nails with plastic collation. If you are using Clipped Head nails with the paper tape collation the angle of the nails not let the driver catch the next nail and drive it into your work.
If you have the correct nails, check the magazine to make sure the nails are feeding all the way up to the driver. If there is anything blocking the way or if the spring on the feeder isn't taught enough to pull the clip all the way up the driver won't be able to catch the next nail and drive it in.

I would look at the nail collation and nail angle first. If both OK for this tool, remove all the nails and air hose. Bounce the tip and see if the driver falls down on its own and if it does the o ring is worn and would need replaced. When inside the tool look for other wear items.

I would check for the proper nail guage and collation and if that is correct I would take it back for a replacement. But check the nails first.

could be couple problems first make sure using the right type of nails I.E. collation and degree of angle. Next look for jammed nail in the nose area and then look at the nose condition for excessive wear and lastly look at the tip of the driver for wear and not being square so it drives the nail straight.

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if thats a hitachi nr90ad, then you should be using 30 degree paper taped clipped headed nails,, never use plastic or wire collated nails in that gun,, some boxes might say 30 up to 34 degree and thats fine, also in a lot of areas clipped headed nails are no longer code, so they made the nails full headed, the head is just off set,,

any full round head with a 22 degree angle and plastic collated nail between 2 to 3 1/2 inch should work, they don't list any other specific brand but these are the requirements for the tool.

if memory serves me, Senco brand nails, and nails with the same angle, will work

The F28WW takes 28Deg nails that are collated with wire, so your nails won't work in that gun. The N80SB is also a 28Deg nailer so your 30 Deg nails won't work in that gun either. Sorry about the bad news but it looks like your going to have to spring for new nails

Yes you can generally interchange the two (paper vs plastic collated) as long as the nails meet the specifications (and collated angle) specified by your nail gun.

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