Mods wont install

[DemonCrusher36]

I run the server, I can get on fine but another person trying to get on it cant get in.

We've tried:

Reinstalling tekkit (clearing cache)

Deleting the tekkit folder

Reinstalling many times.

Yes, its on the correct version (recomended) and it still wont work.

Also tried the usual fix, any idea?

Also, the mods folder actually has all the required mods.