I had the same problem last night when I attempted physics for the first time... I decalred all the variables needed, but the darn ball wouldn't fall!
I tried adding a z++ to it, and as soon as the ball came near to the ground, the physics engine kicked in.
I then removed the z++ feature, and moved my ball as close to the ground as I could, and the engine started up just right. Physics and all...
So if your model is a bit off the ground, try moving it down a bit. Might work, might not... but it worked for me...