There are four four-wheeled trucks that ride in the upper and lower tracks.
Those trucks will fail, losing a wheel, making the door difficult to open.
Replacement trucks may be purchased to restore normal operation.
Note that there are three sizes (heights) of trucks, so use the correct parts or they will not work.
First step, make sure that there isn't crud in the tracks. I use garden hose fresh water, after vacuuming to keep mine clean.
Good luck w/ your new boat.
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