When adjusting media flow, it is possible to over extend the outlet pipe. When this happens, you may not be able to get the abrasive control lever back into the locked position. As you can see here, the lever is stuck up, and won't settle back down to the locked position as it should.
This is because the outlet pipe, inside the machine, has become misaligned, and won't seat properly on the inlet jet.
You can often fix this by swiveling the head of the machine. Grab the head of the machine and wiggle it back and forth until the outlet pipe falls back into place.
If that doesn't work, you can fix this by removing the bolts, ears and bottom flange from the machine, and re-seating the inlet jet properly. Make sure you don't lose the gasket that sits above the inlet jet during this process.
Put everything back the way it was, and you're done!