4x4 problem

[mxracer511]

So I was doin a lil off road exploring tonight and went to go over a log and put it in 4 backed up a bit and went forward again to give it a chance to engage and pushed up against the log and neither front wheel did anything that's in the auto position both low and hi range I got out locked the hubs and it engages fine any ideas on why I've never messed around with this type of stuff much