Have the same issue with our 2011 AF 1150. Solution was to take the insert and put a solid 1*4 piece of wood on the bottom of it with screws. The problem is the hinge keeps getting looser and looser and once it moves (daughter turns over in the night) over a hair it drops. The solid piece of 1*4 under the insert makes the whole thing one solid unit. So now even if it moves over it will not drop the middle. The other option is to create new insert with 3/4'' plywood and foam.
Downside is it will not fold up anymore for storage. So we just keep it under our bed for traveling now.