I had this the first time I opened it and realised after a while thinking I had broken it that you have to slam it harder than you think to latch it in and keep a hand either side as there is a latch each side.
The bonnet closes properly on older models if you just drop it from 30 to 40 centimetres height. The W212 (or the W204) does not, you have to help with hands as mentioned above. Just be careful to avoid dings when slamming too hard.
If this does not help, the lock/buffer needs to be adjusted.
Resurrecting an old post here but thought I'd add my experience in case anyone else having a similar problem.
I couldn't shut the bonnet properly on my W204, wouldn't latch on the offside even with very firm hand pressure and letting the bonnet drop. Didn't want to push really hard for fear of denting it.
I ended up adjusting the hook (U-shaped piece that the catch grabs hold of) on the underside of the bonnet, just by a few turns clockwise on the nut to increase the length that it protrudes down. A bit of WD40 on the catches as well and bonnet is closing and latching perfectly