My strong suspiscion is that you're using small moulded pieces (callets), and that it was not freshly produced (it has some age on it i suspect). my guess is what you're seeing is the result of the callets picking up ambient moisture, and subsequnetly thickening. depending on how much moisture it's picked up, you may be able to com pensate by adding 0.1% fluid lecithin to it and letting it mix vigorously at 120F for 15 minutes. If that helps, but doesn't get you all the way there, you can try adding an additional 0.1% lecithin - however that may actually make it worse. lecithin works up to a point, after which it has the opposite effect.
Alternateitively, what you may be seeing is the result of heat abuse (getting it too hot). if that's what it is, i'm afraid there is no fix.