Hi Ian! Got your PM, but thought I’d write you here.
It’s actually simple to remove. Just hit it against the floor in this way. -
As crude as it is, that is the standard method for removing sao from the dou.
To make sure your sao is facing the right way, take a look around the square hole of the dou. You will see an indentation of where the sao was initially fitted on. One side is curved which matches the bottom of the sao. Likewise, the other side has a straight indentation which also corresponds with the upper side of the sao.
Even then, you might find the sao won’t fully fit in. It needs to be flush but that’s totally normal. Because the wood in that joint naturally shrinks over time, the craftsman make that joint incredibly super tight so it will balance out later on. It frustrates me though. It took me about a week of jamming the sao in, waiting for the wood to compress, and then jamming more until it finally fully fit in. However, mine seemed especially tight. Most people need to wait only a few days.
Because you will need to force the sao into the dou, start with the sao fully assembled so that you can grip it well. Here’s a video of someone setting up their shamisen. - https://www.youtube.com/watch?v=yGTBS7CHi7g
And yes, don’t be afraid to drop your weight down on it. It does take force to get it in. When it refused to budge, leave it overnight. Then, pop out the sao and try again. It should lower a bit further. Hopefully all the way until it’s flush. For me, it lowered a few millimeters at a time.