Non Isofix Childseat fitment to C Class?

[R3BB]

I'm coming from a car that doesn't have Isofix, so my childseat is a seatbelt type.

I'm trying to fit it into the back of my 2012 C Class, but the seat belts in this will not lock.

Am I doing something wrong???

 

[whitenemesis]

Try pulling the belt all the way out and then slowly allowing it to reel back part way back in. Does it lock if you then try to pull it out again?

 

[R3BB]

No.
It doesn't lock then rachet back if I pull it all the way out like my old car.
It will only lock if I jerk it, but then goes loose again (like normal seatbelt operation).