I work part-time as a waitress in a restaurant, I am paid NMW. I've been counting my wages for 4 weeks & they've been exactly minimum wage.
I know that we receive tips because I'm literally given them and I put them in the tips box. I've also had a word with a colleague and he's confirmed that he doesn't get his tips either. 
I don't want to cause any tension but WIBU to ask my manager about this?