Why is it that no matter how much Vodka I drink, I never seem to get drunk?

Whenever I drink bloody marys, screwdrivers, even straight up shots, I hardly feel affected. I'm drinking good stuff too, not cheapo brands. I've actually counted fifteen drinks (not including shots) in one night and the most that happened was I felt buzzed at one point. Instead, I get very tired and feel warm. The same happens to my friend. How common is this reaction to Vodka?