It could be possible that you have a minute nick or ding in the car track. With a clean stowed boom, run your hand and fingers down the whole length of the car track and check for any minute dings, burrs or dents. A loaded car will hang up when it encounters one whereas when unloaded the car zips on by.
- - Depending upon the car design you might be getting some "lifting" of the car that angles the car to the track which will encourage the car to "dig in" to the track. Some cars have "inserts" to aid in the ability of the car to slide. Those inserts wear out and even absorb salt
and start to generate considerable friction when loaded. Periodically I have to take my Antal car off and wash them with soap and fresh water
to get the salt
crystals out of the slide track area. Then I use some dry teflon spray to "lube" them.