front diff wont lock under heavy acceleration

well i found out this past weekend when i got into some very very deep mud that if i lightly get on the throttle i can get all 4 tires to spin. but when i wack the throttle to get some tire spin, the fronts will not move at all.
i did not have any warning lights come on or anything like that.
does the front diff actuator have a power limit?
are the black waters too heavy when in deep thick mud to keep that front diff locked?
thanks for your help guys.