I have been having problems with the back end of this fin popping out. It is a nice snug fit when I put it into the futures box, but twice now has popped out. As I can't see how the back is coming out I can only think that somehow over a session its moving itself forward until the roller is in line with the entry gap in the middle. Only trouble there is that you think I'd notice the difference in board performance as it slide forward. Anyone had this problem and maybe a fix it solution. Its on a deep 10'1" x 30" . The fin is the fat boy I use as a single. Works really well with the board but concerned I'm going to lose it. Cheers

dont know much about it,but FCS fin -FUTURES box ?
could this b the problem?

It happens on some boards. It may be that the box have been over-sanded, or deformed slightly on resin curing, or something is preventing the roller-on-a-pin to engage fully into the lateral ridge of the US box.

You may try checking for any resin droplets on the bottom of the box preventing a tight fit, or try the fin in another box to see if it sits deeper and that the depth is really the issue there, in which case, sanding the fin heel may work.