I might know the problem... the day i got my slim i had a quick peek inside by looking through the open UMD door, on the bottom, there is a small white strip kept there for outside elements not getting inside the psp (im pretty sure thats what its for =/). anyway, check inside the bottom of your UMD door to see if that "white strip" is still there, if not, that may be the problem.
EDIT: Ah ha!!! i see the problem! i looked at the second pic and saw that the white stip is crooked. you may have to carefully place that back to the bottom of the UMD door. if you have any problems, tell me!