My girlfriend is a waitress and I am not going to give her a tip.
Why don't you want to tip your girlfriend?
She waited 20 minutes to bring the check and drinks.
Wait staff have jobs that are often impossibly demanding. Many things that the customer sees as "bad service" are out of the server's control. Computers go down, so you can't print checks; cooks show up drunk -- or not at all -- and the food suffers.
How do you expect me to tip her then?
You could always do something nice for her when she gets home as a tip.