We went there last night; the food was ok but not ...

We went there last night; the food was ok but not great and the service was fine, but from three or maybe four different people. When I paid, I was asked straight up for a tip. Now, I'm a tipper, but I hate to be asked and usually say no if asked outright. This time, with good grace, I added $20 to a $180 bill but was taken aback when the waitress did not say thank you or even acknowledge it with a flicker of a smile. I'll never go back.