The fuel stays in the back of the tank because of inertia. In fact, I prefer to add a small bit of stiff tubing in the pick up line so that it can't move to the front of the tank. This will eliminate the clunk from getting stuck into the front of the tank. See attached diagram to see what I'm talking about.

A common mistake a lot of beginner's make is while on the ground they will hold the nose of the plane down to check their needle settings. Doing this on the ground can and will cause the motor to lose fuel pickup because the fuel will run to the front of the tank. This because the plane doesn't have any forward momentum and therefore doesn't have any inertia. They will then assume that the same problem exists in the air, which it doesn't because of the motion of the plane.

Hope this helps

Ken