The string wraps around a plastic spool inside the head, which can tangle, come loose on the reel or get clogged, causing the line advancement to stop working properly. When the string no longer stays wrapped tightly around the spool, the system will stop working; conversely, if the string is wrapped too tight, the string will also not advance properly. When replacing the string, always wrap the string in even, parallel rows, making sure the lines don't crisscross or overlap each other.
As the string revolves around the head, cutting down weeds, a small section of those grasses can get sucked into the head and clog the line advancement system, which usually occurs after you've trimmed long, tall grasses, as the head needs to work harder to clip off the grasses. For this reason, Homelite recommends cutting tall grasses from the top down. Examine the head, spool and inside the head for any buildup or weed clogs, and clear it out with your fingers or a knife if necessary.
The spool that holds the cutting string must move inside and with the head. If the spool no longer spins properly inside the head, the line advancing system will stop working. Since these spools are made of plastic, they may break easily when tapping out more cutting line. Homelite recommends tapping the head only once to get more line out. If no line comes out, stop the trimmer and locate the source of the problem rather than continuously tapping the head harder and harder against the ground.
The spring underneath the spool can also wear out from over-tapping. If the spring has lost its elasticity, the force to push more line out will not be strong enough. Also, if the spring doesn't properly engage the spool inside the head, it will not do its job. If the spring falls out during string replacement, carefully reinstall it before putting the loaded spool back into the head. Replace the spring if its tips are broken or bent severely.