Rail And Slider
WARNING: This page is about a different car, the 2014 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.
Pull the emergency lever to check that the temporary stop lock mechanism operates properly.
Move the ball stud bolt along the rail to check for play or stuck.
Replace the power rear gate drive unit (PRG D/U), if it is found defective.