Custom materials not working

I run ttt server where there is some custom content.

the weapons work fine but the icons with a picture of the weapon aren’t working.
they are in /garrysmod/gamemodes/terrortown/content/materials/vgui/ttt
they always download when you join but they still act like they don’t exist at all.
even weirder, some people say they work fine while others (including me) just have a purple and black checkerboard, though these people may have gotten them from other servers.
i think i had this problem before and after GM13

please help me.