Inspect the towel rack and determine the cause of the problem. Check the holders on each side and see if either is loose. This is the most-common issue.
Grip the holder and slide it up to release it from the bracket underneath. Remove the bracket with a power drill. Loosen all of the screws holding it in place and pull it out from the wall.
Attach a 1/4-inch drill bit to a power drill and make the current hole in the wall bigger. This will create more room for the new bolt and anchor to be installed. Repeat the process for the other hole.
Slide a bolt through the hole in the bracket and attach a butterfly anchor to the back of the bolt. Repeat the process for the other hole. Collapse the butterfly anchors in the back of the brackets and slide them through the pre-drilled holes.
Drill the bolts into place with a power drill until the bracket is flush against the wall. Slide the holder back over the bracket and attach the bar to the holder.