I do not think your problem is keeping still.

Maybe you have stayed up to long beforehand, you still need to be pretty tired when you go to bed. Also you need to relax before as well. Anyone of these could stop you from WILDing.

If you stay up to long in the night you will find it impossible to go back to sleep. Another reason you might be failing is because you are thinking about what you are doing/waiting for sleep paralysis to hit. If you think about it, it just takes longer to come, so just focus on your breathing, don't worry about sleep paralysis or time or anything.