It is common for one TV to mount fine with 25mm long (1 inch) screws while some other TV needs shorter or longer M6 screws. They always are M6, but lengths do vary, so no surprise. I would make sure that when you put the M6 screw into the hole in the TV, push it in as far as it goes. When it stops... that is the threaded insert in the TV cabinet. If you hit that, you want 3/16" to 1/4" of threads sticking out of the hole before the head of the M6 screw. If you have too much screw sticking out, you need shorter screws, If you never hit the threaded insert in the cabinet, your M6 screw is too short. A "threaded insert" is a lot like a nut, without the hex-head shape. These are molded right into the plastic so you have metal threads in the plastic shell of the TV. These are large enough to stay in place when you install mounting brackets, even the ones where you can pull-push/raise-lower or rotate the TV. It would have been nice if the VESA standard also specified screw length so one screw length would work for all TVs... but it didn't come out that way.