The operational clearance between the piston and bore is very tight. I would try putting some known good penatrating oil in there and let it sit for a day at least before trying to move it.

You should visually notice if it is stuck at the top of the bore by turning the engine over with the delivery valve out. If it is moving examine the delivery valve carefully. If it is staying open no pressure will be developed I believe. Or inadaquate pressure.

A stuck element piston is not that unusual with a pump out of service. You just do not want to chance more damage to the precision fit any more than needed. Hence the soaking with penatrating oil.

When a diesel injection pump is dry and has been sitting around with no storage chemical in it. I do not want it particularily. Preffer a pump off a wreck still full of fuel.

This does not gaurantee anything of course. I just feel the odds of troubles like you are experiencing are less.