it's the locking mechanism that needs to be replaced. I had mine replaced due to the same issue, unfortunately out of warranty. The plunger that pushes and pulls the locking mechanish breaks, probably due to material fatigue. It is made out of plastic, one wonders why MB makes such a bad decision in the choice of material. Anyhow, it requires 2 hours of labor, the part costs about $80. If you are handy enough, you can save some $$$. I have looked into the procedure of removing the hatch door panel, it's kinda PITA.
steve