Should I do something (bartending question)?

okay so for my summer job I work in a cafe (in a gym) and during the night we have a small bar where we serve beer and wine. last week I sold this group of ladies 4 bottles of wine and they did not tip me a single penny ( I even went outside to get them a bucket of ice to chill the wine). I was really nice to them and they are all loaded with money. I usually make about 20$ in tips doing the bar, but again I made 0$ last week. I am doing the same bar tonight (with the same people I presume) and was wondering if there is something I should do or should I just grin and bear it? I'm not gonna spit in their stuff or do anything crazy like that, but can anyone think of anything clever I should do? Or maybe anything I could do to actually get tips of these stingy ladies?