I am not so good with injector pumps so others here will correct me if I get some of the terminology wrong.
If you open the top cover on the pump you will see the shut off shaft #4 moves the shut off bar #3.
The shut off bar in turn moves the metering valve #10.
Twice now on two different pumps the metering valve was stuck. I had to open the top cover and free it up. It is all very delicate in there so you want to tread lightly.
Once the pump had only been sitting for a year and another time it had sat for many years.
Both times I was able to open the cover and free up the metering valve and the tractor started right up after that.