The reason...
...that continuous loop furlers (you will notice that they are not sold as "reefers") have trouble holding is because as the line ages and stretches, the diameter of the line reduces. As the line passes over the drum, the teeth won't hold the bite to prevent the furler from unfurling. That was my problem. Also this occurred when I tried to furl the sail in the wind. Some people have said that if you pull both lines so that the line grips on the roller and cleat it off that way, then this will hold. It will, but I have only had limited success that way. I bought a single line reefer because of that. Call Hood, they will offer a deal on a SL drum.