If you give some specifics, we might be able to give some suggestions.
I have found Google image search to be problematic to filter, especially in Chrome.
Sometime SafeSearch is enforced, sometimes not. I found it' s easier to filter on " safe=off" in the URL, but that' s not perfect either.
I have never had a problem with it not blocking specific websites that were in a banned category, though. That has been rock solid for us.
Of course, there are tons of proxy sites. That is a constant battle, but there are ways to fix that, too. Twitter is a big resource for those.
Fortigate 600C 5.0.12, 111C 5.0.2