It looks like that you would like to move the piston & mass so the pressure in Chamber A of the actuator will increase and you are trying to do so by providing an input to the p port. Is that correct?
If so, the answer is that the p port isn't going to drive the piston, it's just for setting the initial position of the piston. This p port is only enabled when you select the following option in the Initial Condistions section.
That said, the solution can be simple: you may add a translational source, force or velocity, to the R port of the actuator. It will actually move the piston. You do need to be careful with the direction of the source and the actuator. The way they appear on canvas (left vs right) doesn't imply sign of the action (positive/negative).
