Answer: Hostas are shade loving plants. So your partial sun location may be too much sun --it depends what kind of hosta you have and how much sun at what time of day. The blue hostas prefer the most shade and do not tolerate much direct sun -- for instance only early in the morning when it is less intense might be tolerated. So if the plants that are performing poorly are blue hostas then I would suspect they are in way too much sun.
Golden hostas on the other hand like to have some direct sun for the best color to develop. Green and variegated hostas for the most part will tolerate a few hours of direct morning sun or bright dappled light all day, but for most hostas noontime sun or hot afternoon sun is just too intense.
Other than that, overfertilizing can cause root problems resulting in foliage problems, as can underwatering (if they are near thirsty tree roots for example); accidental herbicide damage can cause that kind of symptom, too. Sometimes there are unusual soil conditions that can cause problems, so if none of the above seem to apply you might want to run some basic soil tests and see if that reveals anything.
I hope this helps you trouble shoot. If you think they are in too much sun you can move them now to a shadier spot. Just be sure to keep the soil evenly moist for the rest of the summer while they become established.
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