Is it poor service from your server, or something beyond her/his control?
When I waitressed there were a few times I was stiffed on Sunday mornings because for some reason it took f-o-r-e-v-e-r for people to get their breakfast if we were busy. When I saw they were getting antsy I would go back and check, and then let the customer know what the problem was, or approx. how much longer it would be. I still gave them excellent service in drink refills, removing their plates, getting their check to them promptly, etc. yet sometimes because they were angry that it "took so long" they would stiff me on the tip. Not really my fault.
And one night of the week I waitressed alone. No other waitresses on the shift that night. Most times it was fine, not usually a very busy night. But sometimes we were slammed. I could be waiting on 40-50 people, by myself. And bussing tables. And running the cash register. And answering the phone and taking to-go orders. Most people were SUPER understanding, and tipped me well even if I couldn't take super good care of them. But there were occasionally people who would not tip, and acted like they were the only table I had to wait on. Could they not see I was running around like a chicken with my head cut off? I was not standing behind the counter chatting with the dishwasher, or yapping on my cell phone. Would you still not tip your server in a situation like that?
Now if I just chose not to give good service to a customer, if we weren't busy and I just wasn't attentive to them, let their glasses sit empty for a long time, etc. then you would most definitely be justified in not tipping. But I just wonder if you still don't tip even if the problem was not something the server could control?