Gypsum has pretty low solubility in water. That is the reason powdered gypsum is preferred - due to the massive increase in surface area to be contacted by water.

Do you still have a few pellets? You might try a jar test where you put in a chunk of a bulk pellet that was roughly equal in size to your one pinch that worked previously.

If your pond is not actively accumulating lots of silt or organic muck that would sequester away your gypsum pellets, then you may have created more of a long-term buffer for your pond water. And hopefully your cost and effort still yield you some useful benefits.