Mine does the same thing but I'm so used to this common occurance on many cars not just MB's
Alot of times its mostly pressure of the steering column pressing against the tumbler, sometimes it happens all the time sometimes it dont happen depends on the surface your cars on Best thing I found that worked other than replacing a tumbler which may or may not solve the problem is to just relieve some pressure from the wheel before turning the key
meaning turn the steering wheel to the right till it stops which is just a little more than a hair then turn the key you should have no problems.

If it still dont unlock dont force it just wiggle the wheel a little more my locks been like that for years and I have no problems getting it unlocked that way


Jeff