[SOLVED]Collision box "glued" to one another?

I created a simple scene to play with OgreNewt.
I applied gravity to each object except the gound(whose mass
is 0), and if I press SPACE, I changed the force from (0,-9.8,0)
to (0,9.8,0) to make everything move up. If I release SPACE everything fall down...
The problem is, after some up and down, I noticed some objects are "glued" to ground and will not move anymore. Unless
they are hitted by other object, they keep still. And these objects are random.
I am not sure if this is a bug or I did something wrong. Many thanks!

I am not sure if addForce() is in local space. If it is then when an object is upside down, adding an up force will have no effect? I have tried addGlobalForce(force, Vector3(0,0,0)) but everything started to rotate like crazy...

I found this is caused by freeze...

pBody->setAutoFreeze(false);


works fine!