DLC-7 missing a piece?
The workbowl has a spring-loaded assembly inside. You can see it through the plastic. When the workbowl cover is in place and locked, it depresses the button on the base. If this is not happening, then you need to determine whether the workbowl or the cover is at fault. Once you figure that out, you need to replace the faulty one. I usually get my replacement parts at GourmetDepot.com.