Getting drunk is a game of roulette?

Basically, to keep things short; some days when I (18F) drink I get super drunk off a few shots, but other days I can do the same amount of shots and feel absolutely nothing. Eating doesn’t seem to be a factor in this, as it doesn’t change the outcome at all. It’s always just a game of will I or won’t I get drunk. Does anyone know why this happens to me?