Texture Replacement

[Bit Strange]

I've managed to replace some textures of a game called Platypus, but for some reason some of the bigger replacement textures revert back to the original ones at random times for no apparent reason, sometimes they even flicker between the two textures. Why?
I've messed around with the graphical settings for about an hour, but to little avail.
Most of the textures have the dimensions of 1024x512. That can't be it, can it?
There's one texture that's 1024x512 that doesn't flicker or completely revert TOO often, but then there's another texture of the same dimensions but is far less in size, yet for some reason it very, very, very rarely ever shows up.

Could someone please help me with this issue? I would appreciate it.