You bump the trimmer head on the grass and nothing happens. No fresh line comes out, or it spits a tangle and stops, and there you are in half-cut grass holding a tool that will not cooperate. On Ryobi ONE+ trimmers this is almost always the spool or the line, not the motor. Work through these in order and you will usually have it cutting again in a few minutes.
First, the line has probably welded to itself
This is the most common cause by a wide margin. Friction generates heat at the head, and the heat fuses adjacent wraps of line into a solid lump that cannot pay out. Pop the head open, find where the wraps are stuck together, free them, and pull a few inches of line out past the eyelet before you close it up. To stop it happening again, do not hold the bump on a fast-spinning head, and wind the spool a little less tightly.
You might be running the wrong line size
Ryobi heads are built for a specific line diameter. Go too thick and it jams or refuses to advance, and you can chew up the spool or spring in the process. Check what your model is rated for. Many ONE+ bump-feed heads use 0.080 inch twisted line, while some take 0.065 inch, so match the number your head calls for rather than whatever is on the shelf. Twisted versus round line matters far less than getting the diameter right.
The line is cross-wound or wound backward
Spools have an arrow molded into them showing the wind direction, and ignoring it almost guarantees a feeding problem. Rewind so the line follows that arrow, keep the two strands in their separate channels, and wind them tight and parallel with no overlap. Cross-wrapped line snags on itself and stops dead. Seat the loose ends in the retaining notches so they do not unravel before you reinstall the spool.
Clear debris out of the head
Grass, mud, and grit pack in around the spool and the retainer and physically block the spool from moving. Take the spool out, clean everything, and confirm the spool slides and spins freely in the housing before you put it back.
Check the bump knob and its spring
On a bump-feed head, the spring under the knob is what releases line each time you tap. If that spring is weak, kinked, or broken, no line comes out no matter how hard you bump. Inspect it and replace it if it has lost its tension. While you are at it, only bump on hard, flat ground. Soft dirt absorbs the tap and the mechanism never triggers.
Make sure the retainer bolt is not seized
The bolt inside the retainer cap, usually the red one, has to stay loose so the spool can travel in and out as the spring pushes it. If that bolt has fused tight, the spool cannot move and the line will not feed. Free it up, and replace the cap if it will not loosen.
If yours is an auto-feed head
Some ONE+ trimmers use a Reel-Easy+ auto-feed head instead of bump feed. If the auto mechanism itself has failed, you usually replace the whole head assembly rather than try to rebuild the internals, and it is an inexpensive part.
Knowing when to stop fixing and start replacing saves time. A spool that is cracked, warped, or melted, or a spring that no longer springs, calls for a new spool or head, and both are cheap. But if the line, the winding, the size, and the spring all check out and it still will not feed, swap the head and move on.