Usually this has to do with the the line pawl and the worm gear. Basically, with level winds/baitcasters a little piece called the pawl guides the reel back and forth along what is called the worm gear. The worm gear looks like a bunch of X's/V's/Cross-hatches. If the pawl is worn out or is not putting enough pressure on the worm gear, the gear will not grab the line pawl and move the reel across to spool your reel evenly. To fix this, there should be a little piece you can tighten, which basically pushes the pawl closer to the worm gear so that the gear grabs the pawl (picture the pawl as if it is little teeth that catch in the grooves of the worm gear). Another trick if you cannot tighten the pawl anymore is to take the cap off of the pawl, put a piece of paper in between it and the pawl, and put the cap back on (what this does is pushes the pawl teeth towards the grooves so they will grab the worm gear).
