I would guess that its engaging the front differential but not the hubs, does it work if you manually lock the front hubs?
You have a vaccum leak would be my guess. One of the vaccum hoses is cracked or disconnected, or one of the hub seals is bad. They make a vaccum system tester that checks for leaks but I would visually inspect the system from the vaccum pump to the hubs. Vaccum leaks are a PITA and a lot of people end up just replacing all the hoses.
Personally I just manually lock my hubs though.
Sent from AutoGuide.com Free App