Question for the Waiters/Waitresses

I've seen a few tipping topics on here lately and I was just wondering something. I always tip. No matter what. I've only ever not tipped once in my life because the waitresses literally snatched my bill out of my hand when I pointed out she charged me for a drink when I got water. That being said, I always tip between 18-20%. More if I can afford  it. However, when I go somewhere for lunch, my bill isn't that high. Maybe $10. I feel bad for only leaving $2. Should I leave more? I feel like waitresses who work the lunch shift probably don't get as much in tips because of this and it makes me feel bad. Should I tip more?