Well, the mechanics are pretty simple. With the machine off I would slowly roll the gantry back and forth to feel for a bump or snag on that side. That way you can figure out where, and then why. If you are out of warranty it may help to pull the top on that side for access.
Open the front and look up under the left side as you pull it forward. Someone in the past has had an issue where some wire had sagged over time and was catching on the circuit board.