The sprag clutch rollers are held in a cage with a very thin coiled spring in a slot around the circumference. When the sprag turns in a particular direction it locks up, in the other direction it just spins. The spring needs to supply the proper tension or else it slips in either direction.
One fix is to tighten-up the spring. Some have done this quite sucessfully and saved the expense of a new sprag. I suggest you just replaced it with new unit.
[Edited on 1-7-2005 by Shazaam!]