I understand 100%.
Link to your exact steppers will help.
What is your step angle?
Are your belts so tight it prevents movement?
Well, to be honest, not sure. Maybe? I will check that too. *ninja edit* I can stall by grabbing the pull and no load (have to really grip but I can)
Do they work fine with no belts?
Will check now
When you say move 50 mm do they try and go 100?
Distance APPEARS (keyword here) to be right.
Are you using all three jumpers under you drivers?
Is you power supply large enough?
12V5A like you have in build sheet
What size are your pulleys?
There is hundreds of questions if you didn’t buy the stuff from here.
Understood. hopefully this post helps others but being so many have been built and no problems so far, clearly something I’m doing wrong. 😉