Hi - my name is Paul & I will try to assist you.
There are a few possibilities & I would hope that some of this has already been eliminated but here is what I would have checked from the start:
First of all make sure the screen is correctly cleaned (may sound obvious but sometimes the glass can build up a film of grease) using a good quality TFR (traffic film remover) at your dealer - brake cleaner also works well & thoroughly wash off afterwards with normal car shampoo
It could also be a poor wiper blade & would try a new set of blades
Another issue we see that may not have been addressed is the angle of the wiper arm - Mercedes dealers should have a special tool kit for checking the arm is set to the correct angle - if not then this can cause issues like this. Make sure that the dealer has checked & set the engle correctly.
Best regards, Paul