Look at Apex's videos regarding trigger bar timing adjustment. What can happen is that the loop releases the striker before the striker block is completely out of the way. This will slow down the striker sufficiently to cause intermittent light strikes. I've had to make this adjustment on all of my pistols with Apex FSS kits in them. My fix was to close the loop down, delaying the striker release relative to the movement of the bar that pushes the striker block out of the way.