I posted about this one year ago, this is an update.
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The specific model was an inexpensive $50 USD Black & Decker Grass Hog.
It has worked great, except for one snag. The design is impressively simple, in my opinion. Inside the feeder mechanism there is a white moving lever. That lever beats against another piece of plastic. After the line stopped feeding, I looked around carefully and noticed a tiny deformation at that point where the lever beats against the surrounding piece of plastic. Apparently that deformation was causing it to stick against the other surfaces. So I carefully used a utility knife to smooth the top of that white plastic moving piece. After that, it began feeding just fine again.
Out of curiosity, anyone buy a plastic grass trimmer which did not have a metal grommet for the string? Did the line cut through the plastic?