I haven't been as scientific about it as you've been (in terms of noting whether the grounds are fine or dry), but my worms definitely have a varied approach to grounds. Maybe your original set was damp enough that the heat produced by their decomposition was dissipated through evaporation? It does make sense that the fine grain size would decompose faster and hotter.

My worms sometimes take at least a week before they decide that the grounds are ready for business.

I read somewhere recently that the grounds need to be fairly damp. Makes sense since the worms require high moisture levels. I'd guess the finer grind is actually easier for them to ingest.

Andrew

I've had this happen with the SAME BATCH of coffee grounds, just dispursed over a period of time, the first and third time the worms ate it up, the second, not very fast. Go figure. Lucky them, though~ I will have pumpkin leftovers later today!
Gina