Cyclone sander the plate keeps popping off when I lift the sander off the surface

Here are steps to change paper in Sanders & Sanding Blocks
Step 1 - Lift Front Clamp:
A finishing block sander will have two clamps--one on the front and the other in the rear--and are usually spring operated. Release the front clamp and remove the old paper. Insert the new paper until it hits the spring posts at the back clamp. Make sure it is even with the mounting plate on top of the block.
Step 2 - Fold Paper:
Fold the paper under the plate and clamp, holding the back end down tightly to pull the front end as far as you can to eliminate slack. Secure the front clamp.
Step 3 - Release the Back Clamp:
With the front clamp secure, release the back clamp. This will create slack once again in the paper.
Step 4 - Secure Back:
Now, fold the end of the paper under the plate and clamp, pulling it tightly. Then, press down firmly.

Power Sander:
Replacing a 1/4 sheet sandpaper for a power sander is similar to the process listed above above except you have a little more flexibility in managing the paper.
Step 1 - Clamp an End:
Make sure the sander is not connected to a power source when adding paper. Open 1 end and insert the sheet evenly into the area between the clamp and sander surface. Clamp it shut.
Step 2 - Bend Paper:
Bend the paper so the fee end can be inserted under the arm at the other side of the sander.
Step 3 - Stretch Paper:
Stretch the paper as much as you can before inserting it under the "hold-down" arm. Make sure to get it as tight as possible and push the end in as far as possible. Getting the paper tight is the most important part of the process.

Have you tried the hardware store where the sanders are sold?

Check the wheels on the bottom plate to make sure they are not allowing the bottom to sit flat on the floor. The wheels must keep maintain a gap between the plate and floor or it will suck down like a suction cup.

Maybe you are burning the wood. If the wood is hard (like oak or maple) and the grit is fine (like 100 or higher) then this can only be avoided by NEVER running it still in one place. Keep the sander moving quickly over the surface. In fact, you should start the sander BEFORE you touch it to the wood, and when you do touch the wood you should be landing it like a little airplane on the runway. As long as you keep the sander oriented with the grain you can move it forward and back as well as side to side. Just keep moving until you lift it off of the wood - this also helps keep the surface flatter. Good luck.

the skil 439 is a palm orbital finish sander. To replace the paper, pop the springs off the side where they take a 90 degree turn towards the work surface. To replace paper, rip a piece of standard sand paper into quarters, reverse removal and done. It's a pretty nice older sander and yes I have one.

I would check both rollers for debris on them, also check the plate on the bottom for unusual wear, be careful for sharp edges and make sure the front of the plate is not worn excessively. Then I would turn the front roller without the belt on it to make sure not out of round and moving smoothly. Lastly if you are using directional belts make sure installed properly with the arrow pointing to the front of the sander. Let me know if you need more help before voting and I will see what I can do.

It should just pop off the back.

If the sander was on for a long period of time maybe 20 mins or so then the switch has probably stuck in the off position when you turned off. Try a new switch or test the old one. Try Solent Tools on 023 80578057 for spares or further advice.

Go to this web site, and enter the model # of your sander in the box in center column, near bottom of page.
If this doesn't take you to a manual, enter model number to see parts list, and look in upper left of parts diagram page and there's a link to manuals.

http://www.dewaltservicenet.com/servicenet_home.aspx

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You did not specify how the sanding discs are meant to be attached to your sander's backing pad.  My experience with this type of problem is material from the sanding process builds up on the backing pad and does not allow the sanding discs to adhere properly.  If you have a velcro type system, clean the hooks on the sander's backing bad by soaking the pad's face in a plate of paint thinner for 20 to 30 minutes then use a wire brush to remove the rest of the paint physically.    For adhesive pads clean the sander's backing pad surface with denatured alcohol.  Do not use isopropryl or rubbing alcohol as this may not be 100% alcohol and could contain a lubricant like glycerin.  When the sander's backing pad is completely dry and clean it will hold adhesive backed pads fine.    Flaky paint is best scraped off before sanding, use a wide putty knife or a paint scaper to remove large flakes before sanding.  This will minimize airborne dust and cleanup.    For best results with adhesives, always keep your surfaces  clean!    Good luck and happy sanding!  Michael Mittelsdorf