Anyone know a fix to this problem, worked fine at store and when we got home the gate went down and won't latch either mechanically or manually...

 

Had the same problem few days ago and it drained my battery flat. Sent it back to the dealership and found out that this is a common problem with the ML and you will need to replace the catch.

 

usualy what i do first is...open the tailgate and use a screwdriver or a pen to latch the latch at the bottom on the liftgate (be sure to fully press latch firmly)...the latch should audibly latch and stay. you shouldnt be able to pull it with your finger and have it unlatch. if it latches then the latch is probably ok. if it does latch then be sure to grab the liftgate open handle to unlock the latch so you can close the liftgate.or use unlock on the remote.
could also need a striker adjustment.
also it could be the hinge at the top of the liftgate door. one of the screws can break off and cause the liftgate to not close evenly.thus not latching.