I take Ambien (zolpidem) to sleep at night. I have been taking it for 3 months now. I took it at 9:30pm I'm still awake. It's 12:15am. Why can't I fall asleep?

Maybe not as effect. You may be becoming sensitized meaning it will not work as good. You need to discuss with your doctor other options.
drop the pill. You need evaluation on why pills just make things worse. Talk to a sleep specialist.