Any luck on this?...... sounds like it's moisture related if you are inconsistent, as another user pointed to. Try to put parts in 3 different "Lots" ... First lot put parts in bags sealed tight, and second set, parts add 1/2 cup water and also put in bags, and for last lot just keep them out in the atmosphere. Check these at 10 min, 1 hour, 4 hours, 24 hours, 48 hours, and see your results with different amount of moisture coming back into the parts....... 2nd step, may need to anneal parts.... (but, is your mold temp warm enough as JayDub stated?)...... (So, annealing is heating back up, and cooling down, can find better definition and specifics on internet for your material).... and with annealing, parts will fully crystallize and shrink and stay more uniform from there.

Good luck
Joel