How long are you keeping it warm?
IT is really only necessary to keep it warm for the first few presses in the schedule I use (5 min, 10 min, 20 min, 40 min, 1hour, 2 hour, 4 hour, 8 hour) in order to knit the curd. After that, there is no need and it is in fact not good to keep the cheese warm for the reasons here discussed.
After we get a good knit, the cheese can be allowed to loose its warmth.