Sounds like you are having the same issue I just had with a cheap non Mercedes switch. See post #26.
Jammed ignition lock - steering column mechanism worn
I really had to monkey around with this switch before I got it working. Turns out there was an internal dimension that is off. The switch binds up after you tighten the screws. It needs to be completely disassembled by pushing in the black lock tabs and pulling the switch mechanism out of the housing. Then You need to shim it with three washers inside, between the housing and black switch mechanism.
Note this is not putting washers on the outside. It needs to be taken apart and have the internal clearance shimmed. Then you snap it back together.
That Chinese switch is junk. I will eventually remove it and put in an MB switch I recently ordered once it cools off outside.