I also had the opposite problem when starting WILD but now it seems I have your problem. The thing about WILD'ing is that they are not possible without you actually falling asleep, but you have to re awaken your mind quickly, shortly before entering sleep paralysis, when you get to SP you will know it, its unmistakeable and I doubt you will only experience heaviness. Try staying up for longer before going back to sleep if you are doing a WBTB