Having been sailing race longboards for a long time, I tried a Naish Icon 130litre short fat thing and found I could sail it well on one tack but I was hopeless on the other tack with the board heading up all the time and refusing to bear away. I realise its a fault with my technique but anyone got any ideas precisely what is at fault? Or what I could try to correct it part from the obvious(standing up more, not hanging off the back etc etc). Been using a seat harness, maybe a waist harness might help??

probably not harness. In getting going push through front foot and weight down a bit through mast foot by pressing down a bit. Front foot into strap then when planing a bit put back foot into strap; when you move back foot into strap press down with hands during the transition to keep board flat.