Greetings
CFTraveler is right.
There is no answer to this.
Even more perplexing to my mind is why most experienced OBE'ers sometimes get vibrations and sometimes do not. Sometimes they are in sleep paralysis and sometimes they are not. Sometimes they are very clear and lucid while OOB and sometimes they are not.
This happens to me ALL the time. During one OBE I will get vibrations and be in SP. During another OBE a few DAYS later I will have neither vibrations nor SP. And I read the same thing in Many OBE accounts from some very skilled and experienced OBE'ers.
I have yet to make rhyme or reason from it.

Regards
Grey