Grasp the binding and attempt to move it fore and aft on the track. If it moves, it may not be locked down. This binding must have the lock pin in. The binding length should read the same. This affects not only the boot fit but forward pressure. What you are describing may be either an unadjusted binding or a malfunction that should be looked at. The piston will restrict movement, but if the piston is removed, you can remove the lock pin in the heel and slide the entire binding off the back of the rail.
The dial under the center of the foot is used to adjust the approximate length using the scale in mm on the binding. Then you have to eyeball the forward pressure indicator, low on the center of the heel. There is a triangle on it. That triangle needs to be about halfway buried in the housing, which indicates the right forward pressure. If it is in too far, you have too much pressure, if it is not depressed into the housing, you will release too easily. I generally go for the plunger to be flush with the binding housing to load it up a bit. Once the length and forward pressure are set, the DIN is set and tested. Oh, and you better know to set the binding retainer pin in the vertical (locked) position.