1) make sure you can see a clear path by looking in the driveshaft bellows, through the gimbal bearing and into the coupler. Make sure nothing is visibly obstructing.
2) make sure the height of the outdrive is equal to the bell housing so that the outdrive will go into the bell housing without being either high or low. as you feel the drive shaft wanting to make love to the coupler, you may have to turn the yoke slightly so that the splines line up.
3) you may have to lift the drive shaft/u-joints slightly while the trying to get the drive shaft to mate properly to the coupler. The problem being that the u-joints cause the drive shaft to 'sag' a little bit in the bellows and it raises the end of the drive shaft ever so slightly so that it does not quite mate with the coupler.
Other than that, maybe consider getting an outboard motor

