So the best I can figure, these use a single fixed reference point (the magnet you put on the curtain rod) against the time it takes to travel from the reference point to the user set point of "fully closed curtain".

To troubleshoot yours I'd recommend first removing the curtain. Verify the magnet is positioned where the bot would be when the curtain is fully closed, then set the open position in the app by running it back to the "open" bot position (so it can record the time it takes to travel). Once set up, set it to open, then closed, then open, etc and verify it keeps both its positions. If the bot works in this config, then add the curtain back. If it stops working reliably then the introduction of the curtain is the issue.

Could the curtain be too heavy? Is the curtain rail not providing good grip for the robot? etc.