Answer: Based on your description, I am not certain what is happening but I would investigate the roots first. It is possible that voles are munching underground, they love hosta roots and will move on to other plants, too.
Next, I would think back and wonder if these plants could have been overfertilized or in contact with fresh "hot" manure and are suffering a bit of a reaction to that. If this is the case and it is not too extreme, a little extra watering and time will solve the problem.
Also, could these particular plants be suffering water stress due to over or underwatering? Overflow from the gutter could overwater them, overhanging eaves or branches could limit water reaching their roots, or underground tree or shrub roots could be robbing them of moisture.
Finally, there is a chance of an infection of some sort at work, and if you can't deduce any culture-related problems as mentioned above, then you might want to consult with your county extension and see if they can determine a cause for the problem.
I hope it is something simply solved.
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