Locking doesn't lock from first click
 

Morning lads and gals. Got a 2012 E220 Avantgarde. When I press the locking button on my key fob the doors visually lock but when I try and pull the handle they are unlocked. I have to hold the lock button for like 5 seconds and than press it again so they can lock. That is so annoying. Anyone can help with something? That will be much appreciated
Thanks
Velislav