Assuming you're not fishing...
It sounds like you are stalling the plane. You can't simply point and shoot like you would in a first person shooter, or some very poorly modelled flight sims. You have to know the dynamics of flight and the concept of energy in relation to planes.
In the simplest terms, an airplane needs to be travelling at a minimum speed to keep itself in the air. You go below that speed, and the airplane is no longer flying, but falling. There are other factors that contribute to an aircraft's performance as well. You can induce a high speed stall by violently disrupting the airflow over the control surfaces. The best way to picture this is the flat spin that Goose and Maverick went into when they flew through the jet wash in Top Gun. They certainly weren't going too slow, but the disruption in airflow caused the engines to flame out and the plane to spin uncontrollably.
You can simulate this in AH. Fly your plane straight and level and then yank back and left/right on the stick. Your plane will spin and fall.
There is some really great info on this board, and a lot of websites dedicated to virtual air combat. Search through here and you'll find what you need.
Oh, and if you were fishing, consider me landed. Take your trophy picture, remove the hook and place me back into the pond.
