There are so many variables when I see stone do weird things. Everyone of the answers are totally plausible. I do very little compression packing anymore but when I did I always used plaster in a slurry mix that I would paint on with my finger tip around all of the gingival contours. Rubbing it in and eliminating any chance of a bubble intense areas. The separator also penetrates this material MUCH better than the dense stone in my opinion. I have seen techs use all kind of stupid stuff like debubleizer and seen them slop on the sodium silicate or supersep from the bottom investment and not rinse off the wax. All of these things leave a residue and retards the setting of the stone. Today with IVOCAP and IVOBASE we do things differently with a flow stone and putty but on occasion I still do it old school just to make me feel better about the 40 Brass flasks I have sitting and laughing at me.