Why can't I throw up?

I'm a junior in high school. The last time I threw up was in 2nd grade due to food poisoning. Lately, I've been getting really nauseas and I feel like I'm going to throw up, but I never do and I've tried making myself puke to relieve my nausea by sticking my fingers down my throat, but I still don't puke! It's not like I'm bulimic or I actually want to throw up, I just want to know why I physically can't vomit!