So i finally got around to building my kickstarter maslow. I know i am late to the party but hey i am here now. I have gotten to the point where i need to retract, extend, then tension the belts. At first only 2 belts would retract. I found another thread where Bar suggested increasing the retract current. I did that and they all retracted. I then extended the belts. I am not sure if this matters but it doesn’t look like the belts extended equal distances. They are close but are a few inches different. I then took the sled and mounted it to my vertical frame and attempted to tension the belts. Nothing appears to happen. I am searching for the answer for a while now but so far have not found it. Would be great if someone who knew could tell me 
Ok so good news…i decided to try something and now it appears to be working. Not sure if this was a fluke or if what i did helped…
Previously i had retracted and extended the belts then i powered the device down and went out side where i installed it on the frame and then tried to tension the belts…
I had this idea that maybe tension would not work unless the belts had been retracted and extended in the same session. So i unmounted it from the frame, retracted and extended the belts again, then without powering it down i remounted it to the frame and applied tension…sucess! Hope this helps someone else.
Yes, when it’s powered down, it has no idea where the belts are, so you have to
first retract them to get them to a known distance before you can do anything.
David Lang
Your observation about the belt lengths is also correct. The machine will extend enough belts to reach the anchors and then a little extra. These measurements are based on the calibration, or the frame dimensions you entered if you are about to calibrate. The bottom belts will be slightly longer too, just to make sure they can reach if you have the vertical set up.