I had the same problem, a mate of mine pointed out a stance defect. My issue was that I was too upright (legs too straight) and didn't rake the sail back while putting my weight forward on port tack (just like the other forum guys are saying it was my less favorable side). This greatly reduced my ability to get into a settled position and coax the board upwind without pushing on the tail. When I managed to get it locked down I noticed a big difference.
You should definitely try a seat harness out when on your big sails too mate, personal feeling is that you'll get more performance out of your slalom gear when you do (I've got one you can try if you want, but its "big boy size")
Nicks right too the tide can be a mofo sometimes and really wreck's the uphill performance on some tacks, especially when your heading back to the beach from the bank and trying to stay as high as possible