Full time 4x4 is not working

I have Jeep Quadra-Trac 4x4 and these systems are fully automatic. But they have to detect wheel spin while moving for it to engage. So if you are parked in a bad spot and can't get moving, you have to pull it into low range 4x4 to force it into 4x4 and get moving. Then you have to stop to shift up into 4x4 High. Unfortunately it does not have a manual override for 4x4. Short answer: Your 4x4 is probably working, but you have to be rolling to feel/hear it kick in.