It sounds to me like you picked two surfaces and applied your load curve to both faces. If this were the case it's possible that the load could be cancelling. Do you have one face chosen?
I'm having problems with an analysis on a can end (like a campbell's soup can top).
The analysis is a non-linear study in which I am trying to first apply 40 psi inside....then 25 psi from the outside. I'm using shell elements and cutting the end in half (symmetry).
The problem (in real life)...
Look at your force/pressure input. You should be able to apply a curve. There are options for loading and unloading. You can vary the force/pressure by "time" and get the results you are after. Make sure the end time is the same as your simulation time.