workshop items down loading and not installing

I don’t know why this is happening, but when I download addons for use in Gmod they download but they aren’t installed in Gmod. Is anyone else having the same problem, or to fix it?

Check cash, and if need reinstall gmod, or reinstall steam

what am I suppose to look for in the cache or did you mean cash?

Cache I mean, I am Russian and don’t talk English good.