Hej Philip,
Of this I can speak on the basis of experience. The obstreperous board was probably cut from reaction wood, and will strive to return to its warped status irrespective of the number of flattening procedures. (Firmly believe that wood rejected by the furniture industry and professional cabinet makers has a tendency to end up on the hobbyists’ workbenches.)
Apart cutting my losses, and prepare a fresh piece, there has been some success with a combination of forcing the warping item and sequestering it between two other boards.
Perhaps the skewness can be reduced by screws coming from the aprons (apron screw holes to be elongated)
Shocking with a heavy hammer against a protecting piece of wood during glue-up is very helpful, as demonstrated in several videos on WWMC.
Cauls, while gluing up, can help a lot by forcing the boards flat. If slots for wedges are added above and below especially obnoxious warp, further focal pressure can be achieved. Making the cauls quite tall and a tad convex is also beneficial.
Splines always makes edge jointing easier, and will aid in sequestering the warping board. I have been known to use a Domino router to create mortices for loose tenons.
Finally pieces running in stopped sliding dovetails (inside of the aprons), could perhaps be an alternative on a low table? There’s a lovely serving tray showing this on Gallery Goodness.
Sven-Olof Jansson
London, UK; Boston, MA