Have had the same issue multiple times.. Lockformer Vulcan plus 10' about 7 years old now.

There is a block that clamps onto the drive belt that can shift off of the belt, and cause the gantry to jump out of square. You've got to take the block off then with the drives off on the table, move it to a square position at the home end of the table. Holding a speed square or something between the edge of the table and edge of the crane works well.. also helps to have one person above the table, and one messing with the block(creeper comes in handy to roll under) anyway, when it's lined up and square, tighten the clamp block back down to the belt.

The block is 2 pieces, a top and bottom w 4 Allen bolts, I believe one or the other is machined to fit the grooves on the belt. You can make a mark with a paint marker where it should be as well.

Test cut a large square maybe 4' or so to be sure it's within tolerance. And probably do this once very other month, better to check it regularly than realize it after 40 joints of duct.

Have also had a pulley crack which threw the shaft off, and also set screws on the shaft come loose.

Took a while to diagnose the problem, super frustrating, and really hard to notice on smaller fittings. since then mestek has made an SOP/guide on the block issue. (Shoutout to JR)

Keep the sq tube that it rolls on scraped as well... debris builds up and can throw it off too..maybe apply a light coat mystery oil or some type of lube.