Try different times for WBTB. I set my alarm for after 4.5 to 5 hours of sleep. Staying awake for 15 minutes is short. I use to stay awake for 20 to 35 minutes, most people even longer. However you need to be able to fall asleep after it. If you wake yourself up earlier, it will be easier. I cannot set my alarm after 6 hours of sleep. I would go back to sleep near before our house is busy every morning and I would have problems to return to sleep anyway. I find almost 50% success with WBTB. Find what works for you. If you can wake yourself up after REM naturally, it's good.
PS: I use WBTB for DILD. However I sometimes have WILDs from it.
BTW I noticed there is a new WBTB tutorial here.