What fin do you have in your Serenity?
If you are using the big fin that was supplied with the board, I'd suggest trying
something smaller for your jibes until you get the hang of jibing a board with such
a long waterline.
I found that the best way to jibe the Serenity was to do a flare jibe where you rake the
rig really far upwind (the top of the mast swings well over to the upwind side).
This move the power in the sail way out to the side and drives the board around.
It may be that by trying to go "clew first" (as you would on smaller faster boards with short waterlines) you are actually stalling the outboard drive that would push the nose of the board around.
Also, where are you placing the mast foot.
Usually, on the Serenity, forward is better as this keeps the nose in the water and you get more "nose bite" to help keep the nose heading upwind when you "rail" the board slightly to leeward.
Moving your weight back on the Serenity can work, but it puts you in a narrower and less stable place on the board so coming out of a jibe gets even more trick than if you stay up a little forward of the centerfin where the board is wider.
Try "flaring" your sail well out to the upwind side, and flip your sail just after straight
downwind, well before you would go "clew first".
Hope this helps,