I've had the same problem with WBTBs. I can't really offer any help. I tend to either lay awake for ages or fall asleep - no in-between!
The only thing I can suggest is to try and vary the timing of when you wake for the WBTB, and try to vary the time you stay awake for. Try jst a few seconds all the way out to 1/2 hour of being awake.
I haven't really experimented with that myself, mainly because I have been concentrating on trying to DILD.
I'm now trying WBTB again to give it another go.