The Y zxis of my printer will only move forwards. I can push it back manually and then home the machine, but when I use the computer to manually bring the bed forwards by 100mm this works fine; when I try to use the computer to return it back 100mm it moves between 5 and 15mm then stops, then when I click -Y again it thinks that it is at its 0 point. This started to happen following a print job that was working fine for about 1hour 40min, at this point the X and Y axis jumped out of alignment for no apparent reason and now the Y axis is throwing up this issue. I have printed on the machine several times before without this issue.
Do you see anything related to the Y end-stop in the log window at the bottom? If you do, check your Y-axis end-stop microswitch (wiring/soldering).