I am trying to implement a physics engine into my 2D top-down game and running into a slight issue which I need help with!

I am currently testing out the BEPU physics engine in the XNA C# environment. I have the players and NPC's setup as a "Box" on the server side and adding them to the "Space" to run the simulations every step and that all works fine, but once I have collided with about 4-5 "Boxes" the collision just stop, I can pass right through the boxes as if they are not there. The NPC's are stationary for the moment but the player is moved via the ApplyLinearImpulse call, I am not sure if this is the correct method.

Has anyone ran into the issue of the collisions just stopping or am I doing something wrong? Anyone have a good example of using a physics engine in a 2d top down type game?

Thanks in advance!