My hydrometer flask with the tube the hydrometer came in stuck inside. I have been storing the hydrometer in it with the cap upwards. It fit. Stuffed it in this time the wrong way and the cap stuck.
Tried heating the (plastic) flask up then tapping it upside down with just the cap in to dislodge it. No go. So next I have put a drop of Super Glue on the tube and stuck it in the cap. Maybe then I can yank it out. Wish me luck.