this is a tough problem, it could be vermiculite but I doubt it - even though it holds water it’s still bulky and allows water to drain through quickly. I’ve been experimenting w/ vermiculite and every time I increase it things go well. Currently I"m using about 15% Vermiculite 15% perlite in my peat-based mix.
I’ve noticed that if nutes are off or another problem the leaves droop after watering. My last mix was too hot w/ nutes and minerals and the leaves would droop after watering and stay that way for a long time. The pH of the soil was also too low. Can you tell us what nutrients are in the mix?? *edit, I just checked your other thread, every seems fine, hmmmmm don’t know what causing it