sgmarts hit the nail on the head. If you dislike your bag and currently own a BA pad, the easy solution is buying a BA bag. All BA bags don't include any fill in the bottom half of the bag. Instead, they have a sleeve to accommodate your BA pad. The idea being the pad provides the insulation anyway, so why fill the bottom of the bag with fill you're just going to squash when you lay down on it.
I'm a BA dealer. I would say the two most popular bags are the Lost Ranger (15 degree down) and the Encampment (15 degree synthetic).
I bet that will solve any complaints you have about falling off your pad.