My guess is that you have to concentrate more on the footwork. Contrary to a normal planing board where you push down the inside rail (leeward rail down) to jibe the board, you have to push down the outside rail (windward rail down) to jibe the Serenity or any other board equipped with a centerboard.
While being on a dead downwind course, bank the board so that the windward rail gets down to water. The board starts to turn past the eye of the wind. Being still switch stance, continue to apply pressure to the old windward rail (the new leeward rail) to continue the turning arc. At that time, you can continue the turning arc and go clew first, or switch foot, flip the sail and rake it to the tail to continue the jibing turn to upwind.
You can practice a good exercise by making S curves while going dead downwind and alternatively pressing down the windward rail or the leeward rail without flipping the sail. Once you're confident with this footwork, you can next concentrate on the handwork with sail flipping and further footwork with foot stance switching.