When trying to grip a box, the box just "slips" out of the gripper

edit

As above, I'm trying to grip a box with a robot gripper. The situation is like this.

Try to hold out your arms and make a c shape, bring your 2 hands together, thats what I'm trying to do to a model. But the model "bounces" out of the gripper when any slight contact is made. Can anyone point me in the right direction to resolve this?

PS I'm not using any special collision detectors, just using an effort controller

Thanks

Edit : I've tried to grip a simple plate modeled in fusion360. But whenever the plate is in the gripper, the entire mechanism spasms and the plate flies out of the gripper. I've tried with a small plate made with gazebo's in house box and it can grip, its just that my model has problems... Can anyone please help...

I don't think its a controller problem as I have disabled all controllers and the problem still persists...