clean and grease the mast channels esp if they are new and dry (light coat dont gob it, too much grease will make the rollers slide not roll)

look for bad rollers, ball bearings can fall and stop the mast up by getting stuck between another roller and the channel

the load may be adding friction to the assy

ive had to grease many a mast cause they wouldnt collapse in sequence

It is an older Yale but it's fairly clean and low hours. No work has been done to the mast lately but I'll check to make sure it's well lubed. It only seems to drop out of sequence under a heavy load -strange eh.
thx -Troy

are the channels clean and straight. how old is this truck. has the upright just been repaired maybe shimmed too tight.