Minecraft skins not showing on multiplayer

When I play in single-player mode, my skin is fine and appears as it should. However, when I play on a multiplayer server with my friends it doesn't show up for me, and neither do their skins. However, my friend can see my skin and theirs. We do use mods, but I already had this problem before that. We use the same game version etc so I honestly have no idea what causes this problem.

:)