I have been down that road...new motor..bushings over the years i have rebuilt it 3 times at least. It is the most poorly designed part of the whole outdrive system. The motor is simply underpowered.
As Southkogs mentioned above, first thing to check is to ensure the spring in the hammer coupling is installed correctly to allow the motor to make a quarter turn before it engages the load on the shaft. Then there is the gear case, bearings and bushings They need to be shimmed to ensure no slack or friction. As I recall I had a hard time getting the correct amount of gasket between the cover and the case so as to get proper tollerences inside the gear box, If the tolerance isn't perfect, it will bind the gears inside. Then, the output gear,(on the outside of the gear case) must mesh correctly with the gear on the outdrive. Is there any damage to those parts?
In other words, the electric motor is underpowered...everything must be perfect.
My suggestion is to have someone lift some of the weight off the outdrive as you engage power to the motor. Judging how much weight you must take off might help find where the problem is. In my case, if the boat isn't in the water, I will be lucky if the outdrive will raise 15 degrees and not much more if it is in the water.
What I did was to put the switch near the back of the boat such that if I need to raise the outdrive higher to pull the boat out of the water, I will lasso the outdrive with a rope and lift a few pound off as I engage the motor switch.
Good Luck.