You load a fresh spool, get one pass down the fence line, and the head goes quiet again. Bump it, nothing. Open it up and the line has sheared off flush at the eyelet, third time today. I've watched guys blame the trimmer, swap heads, even return the tool. The trimmer is almost never the problem. Line that keeps breaking comes down to the line itself or where you're putting it, and both are fixable this afternoon.
Old line is brittle line
Here's the one most people have never heard. Trimmer line is nylon, and nylon holds moisture to stay flexible. A spool that's been sitting in a hot garage or shed for a year or two has had the moisture baked out of it, and dry nylon doesn't bend under impact, it shatters. If your line snaps in short pieces instead of wearing down gradually, age is the likely culprit.
The fix costs nothing. Drop the spool, or the whole head, in a bucket of water and leave it 24 hours. Rehydrated line toughens up noticeably. Going forward, buy spools sized to a season or two of actual use instead of the giant contractor donut that'll spend five years drying out on a nail.
Run the diameter your head is rated for
Line diameter is a spec, not a suggestion. Light homeowner heads run 0.065 inch, most standard cordless heads take 0.080, and heavier heads are built for 0.095 and up. Feed thin 0.065 into work the head was never meant for, like thick weeds against a chain-link fence, and it'll snap on every contact. Check the number printed on your head or in the manual and match it. And don't stuff 0.095 into a head rated for 0.080 either; it'll jam, feed badly, and stress the spool spring.
The concrete is eating your line
Every time the spinning tip slaps a sidewalk edge, brick, chain-link, or tree bark, it takes a hit at full speed. A few taps weaken the line, then it lets go at the next contact. If your breakage happens mostly along the driveway or the fence, that's not coincidence, that's cause.
Work those edges with the tip held an inch or two off the hard surface and let the last half inch of line do the cutting. It's slightly slower. It also means loading the spool once a month instead of twice a Saturday. Around tree trunks, back off even more, because bark contact both breaks line and girdles young trees.
Too much line out, or the wrong part of it
The guard on your trimmer has a small cutoff blade for a reason. It keeps the line at the length the head was balanced for. Run without a guard, or with line hanging way past it, and the extra length whips, flexes at the eyelet, and fatigues right where it exits. That's why breaks so often happen flush at the head. Let the cutoff blade set the length, and cut with the outer tip of the line rather than burying the whole head in the weeds.
Check the head for a sharpened eyelet
Line wears grooves in the eyelets over hundreds of hours, and a worn eyelet develops a burred edge that saws the line on every revolution. Pop the spool and run a fingertip around each eyelet. Feel a sharp edge? That's your slow-motion line cutter. Some heads have replaceable eyelets for a couple bucks, and a whole replacement head isn't much more. While you're in there, toss a cracked or warped spool too, because heat-damaged spools weld wraps of line together and cause both feeding and breaking problems.
Buy line worth winding
Bargain-bin line skimps on the polymer blend and UV inhibitors, and it shows. Decent commercial-grade round line from Husqvarna, Oregon, or Echo costs a little more per foot and survives contact that snaps the cheap stuff outright. Round profile is the most break-resistant shape; square and serrated line cuts grass cleaner but gives up some impact strength, so save those for open turf away from hard edges.
Run through this list once and the problem usually stays fixed: fresh line at the right diameter, soaked if it's been sitting, tip kept off the concrete, guard doing its job, eyelets smooth. After that, a break becomes a rare annoyance instead of the whole afternoon.