A brief tutorial on the Bosch Router base
The Bosch base has two adjustments: a lever that does coarse adjustments, and a dial that does fine adjustments. Pressing on the lever allows you to move the router up and down into 3 distinct ranges. Once in one of those three positions you can use the fine adjustment dial to adjust the router bit to the final height. If you find that the lever is getting close to the top or bottom as shown by the red lines, then press it and adjust the router height so that the lever is more towards the center of the adjustment range.
If you crank on the fine adjustment knob too much you can wedge the coarse adjustment lever against the bottom of the housing. This is a common issue, if you don't know about the coarse adjustment feature - an operator will just continue to use the fine adjustment beyond the current range.