In addition to cleaning the mating surfaces well I install my battery bags on the port side of the neck block (for a right handed player) so the bag is not only stuck to the velcro but it's side is also resting the weight of the thing on the side of the neck block when the guitar is in the playing position. Less weight on the bag and less likelihood of it pulling loose.
By the way we have a famous client who damaged his guitar because of a loose battery bag. He took matters into his own hands and tried to super glue the loose bag and ended up gluing the palm of his hand to the top of his one of a kind, signature Martin. Our mission should we have agreed to take it on, and we did....
was to get his skin off the guitar top.... Mission accomplished and Dave got all of his skin off the thing with no damage at all to the one of a kind, serial number 001 instrument.Loose battery bags can be a gateway thing to further damage as said above in this thread.