Why won't my PS2 Slim 75001 play SOME DVD's?

Alongside games, I also like to collect DVD's. Because I am a snob, I like to watch my DVD's on the good old Playstation 2 as god intended. However I have been having an issue where my PS2 will not play some of my DVD's.

At first I thought it has something to do with widescreen DVD's as usually those are the ones that don't work most of the time, but that cant be it because:

A) I can play my widescreen Sweeny Todd DVD and it works fine, albiet shrunken to fit the PS2's aspect ratio

B) The problem extends to some DVD's that are not widescreen, like my copy of Matilda which appears to be in 4:3.

My second theory is that it may have something to do with the speed the disk's are being burned at by the companies that make them. My reasoning for thinking this is that whenever I burn my own CD's or DVD's at a faster speed, my PS2 and other older devices struggle to read them; but I have no way of knowing if this is the cause.

Does anybody else know what may be causing this issue?