Blender Game Problem with Solid Object

Hello everyone,
I’m new at Blender Game Engine and I’m trying to make a Turret Defense Game.
So I have some enemies in the game and I’ve decided to make them look like little tanks.
At layer 6 I have made my tank by using 5 meshes and then joined them with Ctrl+J.
At layer 1 I’ve placed my plane and some empties which are spawning and calling at the same time the tank.
When I’m playing the game, my tanks are sinking into the plane and I really don’t know why… They are already dynamic objects with bounds and normally there should no problem there.
Plz help me
Thank u…