I have the same gun and find it extremely accurate. I must have missed no more than two or three shots with it. Always my fault.
Other than practice, practice and practice a lot more on a fixed target (a plastic bottle hanging under your float) so to get a perfectly clear idea of the ballistics of your gun I can give you 4 more pieces of advice.
1) When you shoot a placed shot at distance, use your wrist to lift the muzzle, in such manner that you will be able to see the spear tip over the chestpad of the gun butt. Otherwise the gun is likely to shoot too low.
2) Always shoot with a stiff arm. No gun will shoot straight if you have a floppy arm.
3) when bass come frontally to you, aim straight frontally between their eyes, don't wait for them to turn. If they offer their side, don't aim to the center of the body: aim half inch behind the gills. If the bass is swimming on his side AND at long distance, aim at his lips.
4) check if the shaft might be bent.
Thanks for the advice, I think some water bottle training is in order and to ensure my arm is straight and grip tight!
Cheers