I'd have a word with the manager and see what could be done. Who said you don't tip well? Why did (s)he say it? And, above all, is it even mandatory in the establishment?
I stopped going to my favourite restaurant for a while because of a similar incident. Here in Brazil we're supposed to tip 10%, and it's even on the bill. I usually pay with my debit card, and this one night I happened to have some money on me. I just went there for some drinks, which totalled R$ 5.00 - R$ 5.50 with the aforementioned 10%. I handed the guy R$ 7.00 and kept waiting for the change (R$ 1.50). It wasn't much, I give you that, but I don't see why you should pay extra to someone who's already doing their job... specially 40%. That's ludicrous.
So, realising the guy wouldn't come back to our table, I called the waiter back and, assuming it was just a mistake, I said he forgot to give me my change. The waiter was visibly angry, and brought me the money without saying a single word and/or looking at my face (yes, he kept the 10%, as expected). Confused, I was rather disappointed that this happened in a place I used to go nearly on a weekly basis. It wasn't a surprise, as all my friends hate the service in that place, but I stop trying to convince them their mozzarella sticks mattered more than the lousy service (the staff is appallingly greedy, going so far as to round up the tip if the total is an odd number and there's an even number of people on the table... and the end result is just one extra cent!).