Wierd that. I had exactly that problem with a 6.5 sail I bought a few years back, and i wont say the brand because it might be seen as sledging them unfairly. It was a reputable brand which was supposed to be exactly the right sail for the board I have. Others who I have talked to with the same sail recon they are great.
But,.. the thing was an absolute pig! It handled exactly like you say. It just never trimmed up right and I was always fighting the bloody thing. I had the "experts" help with the rigging up once when they were down there. They tweaked batten tensions and fiddled with outhauland downhaul, and although it looked a teeny bit better it made no difference.
Anyway, one day I lost one of the many fights I had with it and ended up having a big stack in. I went through the middle panel.
I was so cheesed off with it I did'nt bother to get it fixed even though it was a near new sail.
A year or so later, I was patching up some of my favorite old sails with sail tape and since I had a bit left over, I did a quick sticky tape job on the torn panel of the 6.5, even though I didn't think I would bother using it.
It was a LOT of sticky tape and not a very neat job.
The odd thing is, when I tried it again, it rigged up better and sailed much better.
I came to the conclusion that the original sail must have been a Friday sail or something and was either cut or sown not quite right. Just a few millimetres variation in some of the seams can make a lot of difference to the contour and I think because i taped it up as it lay on the driveway, it ended up a better shape.
