Some have had the problem where the plastic is so brittle, that it breaks apart, and then allows the anchor on the cable that attaches at the top to pull through the bracket. If that is the case, I'd suggest opening a split type lock washer enough to slip the cable into the center, and then crimp it closed. When you're done, it should look something like this.
Then install the bracket like before, with the washer between the cable end and the bracket. That should keep the cable from pulling through.
Others have apparently had some trouble with the bracket popping off. The bracket is held in only by friction, so if it is made too small, or not bent properly, it may not hold as tightly as it should. In that case, you may wish to make a type of clip to hold the bracket on. I bent up a wire coat hanger until it looked like this.
Before pulling the cable tight, insert that clip over the bracket so that each bent end is under the cable. When inserted, it should look like this.
Those are the two problems that I am aware of that people have run in to. Hopefully, one of them will solve your problem.