First, disconnect the big spring. That'll make working on the track much easier.
There should be 2 rollers inside each track. Sometimes one will get spit out if it jumps over the internal stop.
Blow them all out. Wipe off as much of the old grease that you can. There may be a piece of crud in there being held by the grease. Maybe a pebble, a piece of gum wrapper of someone's earring in the way of the roller, peventing the track from moving smoothly or getting full travel.
Check for straightness of the upper & lower 1/2's of the tracks. Eapecially if the truck was ever in an accident.
Use some lithium to grease up the rails where each of the two 1/2's slide against each other. Make sure where the rollers are that they get lots over the length of their travel.