I don't use the Poser Hair Room, on purpose.

However, IIRC, Poser Hair uses Poser's dynamics engine. So, it should have a dynamics hook on it. Check the hair prop object to be sure that dynamics is turned on for that prop. It might be getting switched off and the dynamic properties of Poser's hair stuffs is being... forgotten. (I could be completely off-base with this. I am really not familiar with Poser's Hair Room, so forgive me if I'm just going off on a wild goose chase.)

PS - How are you managing to actually use Poser's Hair Room? Anything I do in there just ends up looking like Link from the Mod Squad shoved a pair of scissors in an electrical outlet, again... :) (IOW - It'd be neat to see what you created with it, since I'm so terrible at it. :) )