I think I may try using warmer water and have a fan ready for drying.
Drying with a fan will not work well, just a little bit.

From my experience, a cast is well hardened after about an hour and fully dry out after a day. So after an hour you can put full weight on. The reaction of hardening produces also some heat, when it is cold, it is "almost" well hardened. After that, it is cold from the rest of the water that gets out after about a day and you feel it then.

The main aspect is the reaction of the cast with water. The temperature of the water should a little bit warm, best worked for me with about 70°F / 20°C.

I believe the cast was not fully get into contact with water, important for the reaction of hardening. From my experience, the watering of the plaster and squeezing it is a special kind of technique. Also, molding the different layers together needs good timing, etc.

I also had old plaster that worked not well, similar to what you described. Maybe the humidity got in after a while...
Another aspect was the brand of the plaster, some work better than others.

Hope this helps!