I've now settled on the aspect that a mast is not a mast. Just because something says the stiffness is 21, doesn't mean its the same as another mast with the same rating. The reason is that there is a tolerance on description. The key point is that NP masts are technically constant curve (as they've previously advertised), but actually they are slightly more bendy in the top, but not enough to be considered a flex top mast. As a result, NP sails work very well with NP masts, but not necessarily with other masts (as I found out). I think the same is true for other manufacturers, if you get the mast it was designed for, it will work, if you don't, then it might, but perhaps only with certain (usually tight) downhaul/outhaul settings. so if you want to be sure, buy the same brand, or try it first!