Ahhhhh, the Porter Cable 505. Dont know why they quit production but its a fine finish sander. Happen to own the Anniversary Edition...came with a nice Oak Case. Its a nice chrome/stainless steel model as opposed to the grey Works like a charm.
Now, here's what I know for sure about that model.
The felt pad is what came stock..its quite sturdy compared to a foam pad. The paper is sliding off because the spring loaded clip is loosing its tension OR its been dropped in the past and landed on a corner. What grit paper are you generally using ??
Dropping this sander ( its pretty beefy and weighs more than others ) can cause a bunch of problems, it crunches a corner and it throws off the action. Many find that the sander begins to rattle and skip all over the place.
As far as the indentation in the center.... of the pad that's not right at all. The foam pad should be replaced with what came with it to begin with...a nice felt pad.
Also, applying to much pressure will eventually ruin the bearings. This was marketed as a " finish sander " with a small swirl pattern. Some think its a belt sander meant to hog off tons of material at a time.
I liked it for 120 through 280. Higher grit papers are a thicker weight. Trying to put more than two complete sheets, stresses the clamping mechanism and eventually it wont firmly hold the paper.
When using mine, I wouldn't put more than two sheets ( four pieces total ) on at a time and I would make darn sure the paper was firmly attached and completely flush with the pad. You have to fold the front edge and rear edge of the paper to assure a snug fit. That way the paper doesn't flop around and chatter on the pad.
This has been one of my favorite finish sanders that aint around anymore..parts are hard to find..they now cost an arm and a leg if you can find a good one !!...Thumbs down to Porter Cable for discontinuing it.