Stoner is correct. The expanding bung should be taking no load in day-to-day use, it is for pre-loading the headset only. The problem lies in the clamping of your stem to your steerer tube. On a carbon steerer the torque on the stem bolts is important as over-tightening can cause steerer ovalisation which can actually cause a less secure connection.
SFN’s are a bad idea in carbon steerer tubes. I’d go as far as to say the Boardman and Cannondale where incorrectly set up.
I work as a bicycle mechanic. That however, doesn’t mean I am always correct 😀
As a first step I suggest buying a torque wrench or taking the bike to a shop that has one. Also worth considering is the stem clamp type and design, plus the area being clamped on the steerer tube – what gap is there between the underneath of the preload cap and the top of the steerer. Can you reduce this with a thinner headset spacer?