Check to see if the nut on the slipper is turning with the spur gear when you rev it and hold it. If the shaft the spur gear on is turning, then your power is being released somewhere other than your slipper.
If it's being released somewhere else, hold it down and watch all your axles when you gun it. See which one is spinning. If the shaft the spur is on is spinning and no other shafts are (including both center shafts), then your trans is hosed.
I just blew my first hex this weekend on my revo with HPI rims... first ever in 4 years of MT bashing. I thought I had finally blown a diff in the revo, so I limped it back to the car. I didn't hear any grinding, so I figured I'd hold it down and see what the deal was. Well, the front tires were both spinning but neither rear was. Upon closer inspection, one of the rear axles was spinning as was the axle nut, which means the hex is no longer a hex.
Kind of sucks as I don't have a spare set of wheels/tires laying around other than 2 sets of maximizers that I gave up on...