The cutting string inside your Ryobi trimmer wraps around a spool, which connects to a spring. When you bump the head during operation this spring pushes on the spool to feed out more string. If the spring has lost its flex or the spool isn't wound properly, the string will stop feeding out properly.
Badly Wound String
Many times the problem of string not feeding out is caused by badly wound string. The replacement string on the spool needs to be wound tightly and evenly. When the string overlaps or is loose on the spool, when the head starts spinning at full speed the string will get tangled and clogged inside the cutting head. The string can also get fused together if it's wound too tight and too much friction builds inside the head. This often happens if you put in too much replacement string or use a string with a larger diameter than recommended in your operator's manual. Trying to wind too much string or oversized string can also cause damage to the spinning head and the spool, which can also cause string-feeding problems.
During normal operation, the spool, which houses the string, can crack or get bent out of shape. If this happens, the spool will no longer spin properly inside the cutting head and the string will stop feeding correctly. Every time you replace the string, wipe off the spool with a rag and inspect it for signs of damage. Replace any broken or cracked spools before they start causing other problems. Also, if you've tapped the head too hard trying to feed out more string, you can cause damage to the spool exacerbating the string feeding problems.
The spring underneath the spool needs to attach to the spool properly. If the spool is bent or broken, it won't connect to the spring. If the broken spool isn't fixed, it can lead to a broken spring. The spring, also over time or through excessive bumping, can lose its springiness and no longer push adequately on the spool to feed out more string. When you have the spool out, disconnect the spring from the cutting head and check its elasticity. If it doesn't give noticeable resistance, replace the spring. Also, if the spring came out during string replacement, and it wasn't installed properly, you'll experience feeding problems.
Clogs or bunches of weeds can get tangled inside the spinning head, preventing the proper rotation, which may result in string feeding problems. Broken cutting heads can also have the same impact. After every string replacement, inspect all cutting head parts and test that they are working properly.