The two piece, hard cases that put the little teeth/tabs between the two halves, tend to make sliding difficult, and stretch the two halves enough to loosen them. You may be able to find a fix online. One of my palm pre's had an annoying front to back looseness that caused a slight rattle when I typed. I got some of those little, stick on, velcro discs, took the soft, fuzzy half, cut to size, and fed it up from the bottom,between the halves, onto the back of the screen half. It solved the rattle, made the slider feel better, and reduced the side-to-side oreo as well.
I will say, that ever since my early days with smartphones, when a Sprint branded case scartched up my screen. The manufacturer had riveted the case together from inside the phone compartment , then put a soft, cheap, fuzzy sticker over the rivets. One of the stickers fell off (they both came off easily) and the rivet scratched up my screen. Sprint considered this normal wear and tear, that their case damaged my phone, and the manufacturer was c