There are gears that the adjustment carriage rides on, and you have loosened things too much and the gears aren't properly engaged, and they are slipping.
Because the entire mass of the router is riding on these gears it is typical with routers mounted upside-down like this that you have to support the body of the router by hand when adjusting up. And the adjustment isn't quite a binary locked/unlocked arrangement. I suspect you have to back off the lock enough to allow free movement of the gears, but not so loose that they skip teeth. Some routers have a positive lock that you engage after you've adjusted the depth with or without an adjustable gear arrangement.
Also, note that there is a different between coarse and fine adjustment. You would adjust your coarse depth to a "0", and then use the fine adjustment to get your final depth. I suspect the coarse adjustment is the one that needs that sweet spot between locked and loose enough to turn without skipping gear teeth. Check your owner's manual to be sure.