Paddling Issue

I've just switched from a 8ft 2" softboard which is incredible easy to paddle to a 6ft 8" shortboard. I can paddle on it but i'm extremely slow, i see other guys around me on shorter boards who just go straight past me with very little effort. Now i would accept it is an issue with technique but i've had 2 different coaches tell me i have a good technique and a strong paddler (this was on a 8ft board). I've tried having my body in different positions on the board but to no joy. I'm quite a big strong bloke, 6 ft weighing 85-90kg but also from doing a lot of swimming and rugby i have huge thighs/legs which hold a lot of my body weight. Could it be a case that the board doesn't suit me and due to a lot of my weight being at the tail of the board that there is too much drag? Or is there something i should be doing or try?