Mechanism Issue with spring definition

All,

I am trying to define a spring in a mechanism similar to a spring plunger. I have a cam connection (with liftoff enabled) at the bottom. When I move the sled down, the spring compresses as it should. However when I travel the opposite direction (so as to allow the spring to de-compress), the spring plunger immediately lifts off the surface and the spring does not decompress. It remains in its previously compressed state. Does anyone have any idI have Creo 2 with MDX and MDO. Gravity is enabled and materials are assigned.