^similar thing happened to me when I had those cranks. I managed to lose the compression bolt about 20 minutes after I put them on, and it definitely wasn't loose. I put it down to the bolt being so short and the threads barely engaging. I put one of these (http://www.wiggle.co.uk/fsa-v-drive-lh-crank-bolt/) in and checked them regularly and it did the trick*
*until the drive side arm came loose from the spindle 6 months after
^similar thing happened to me when I had those cranks. I managed to lose the compression bolt about 20 minutes after I put them on, and it definitely wasn't loose. I put it down to the bolt being so short and the threads barely engaging. I put one of these (http://www.wiggle.co.uk/fsa-v-drive-lh-crank-bolt/) in and checked them regularly and it did the trick*
*until the drive side arm came loose from the spindle 6 months after