4wd shift from stopped position

My company has a fleet of 2005 trailblazers, and at least two of us, that I know of have been stuck in snow or mud, then switch to 4HD, but it seems to do nothing, no front wheel spin or anything. If I switch to A4WD or 4WD before getting stuck it seems to work fine. My question is do the front wheels need to have some motion before the transfer case can do its job? If so why, is it a service issue?

Ron
[email protected]

no, it's on the fly whenever you switch to 4WD or A4WD.