If your towel bar chronically loosens and falls when you load it with wet towels, the problem may be the type of anchor you used to hold the brackets under the bar holders to the wall. If the brackets are loose, the set screws securing the bar holders have nothing to hold onto. What? You didn't use anchors when you installed the towel bar? It's not too late to fix that, and you'll be happy you did, especially if you occasionally use the towel bar for support when climbing out of the bathtub.
Why the Bar Loosens
Although you can't see them, a pair of mounting brackets are all that's holding the towel bar to the wall. Each of these rectangular or oval metal plates has a ridge on its bottom edge, and the set screw in the bottom of the bar holder -- which is the decorative piece that you can see -- wedges underneath it. It's possible for the set screw to back out -- and there's a way to prevent that -- but more often, one or both of the brackets pulls away from the wall. When the bracket is askew, the set screw slips out from under the ridge, and the holder falls -- and all your towels with it.
Most towel racks come with plastic conical wall anchors, but these are the least secure of any wall anchors. They can easily slip right out of the wall, and they often do. Your rack will be more secure if you substitute ribbed anchors for the smooth ones that came with the towel rack, and substituting threaded plastic anchors is even better. If you occasionally use the towel bar for support, though, your best bet is to use plastic toggle anchors. These spread open on the other side of the wall and are virtually impossible to remove without breaking the wall. They aren't the best choice for securing a bookcase, but they're plenty strong enough for a towel rack, and they're easy to install.
Make It Right
To stop your towel rack from falling, you have to remove the brackets and reinstall them with new anchors. This is a five- to 10-minute job.
Unscrew the set screw under each of the towel rack holders and take down the rack. The set screws on most towel racks require a 1/8-inch hex wrench.
Unscrew both brackets from the wall, using a Phillips screwdriver. Extract the existing anchors. An easy way to do this is to partially drive a screw into each anchor and pry the screw with a hammer claw or a pry bar.
Widen each hole enough to allow insertion of a plastic toggle anchor. The drill bit you need to do this should be approximately the same diameter as the wings of the anchor. It's best to make the holes slightly small so you have to drive the anchors in with a hammer.
Tap and anchor into each hole until it's flush with the wall, then screw the towel bar mounting brackets back in place. As you drive the screws, the wings of the anchors will expand and spread out behind the wall.
Mount the towel bar holders onto the brackets and tighten the hex screws.