Zippers are, in our opinion, still the best way to close a frame bag. They provide easy access, can't rub your knees while closed, and have a sleek aesthetic. Most importantly, you don't have to reach through a giant "chimney" of fabric to get to your gear as with a roll-top bag. While we use the best zippers money can buy (completely waterproof and bomber!), they are still susceptible to failure in extreme conditions if not properly cared for. Just like your chain, zippers must be cleaned and lubed occasionally. Zippers do get dirty and clogged up; periodically applying a little Squirt-brand wax chain lube on the zipper teeth will go a long way in keeping your zipper happy.
Another common cause of zipper failure is overstuffing the frame bag and then jerking or pulling with excessive force on the zipper to attempt to close it. The easiest remedy? Don't overstuff the bag. If you just have to try to put more in there than it wants to fit, try loosening the velcro straps around the frame if you feel any resistance on the zipper with a full bag. This will "relax" the bag, and almost always give you the extra volume you need to more easily close the bag.