Sorry, I have been busy these weeks and I just read your question.
I think that the problem could be related with the homing process. As you don’t have sensors to home the motors, GRBL doesn’t have a reference to start the motion. I think you can do a manual homing by placing the robot in the fully stretched upward position and sending the “Kill alarm lock” command ($X).
Here you have the documentation of GRBL commands in case you need it: https://github.com/gnea/grbl/blob/master/doc/markdown/commands.md