It could be rainfall, especially if they are in soil which does not drain water well.

It could be too little sun.

It could be too fertile soil, or too much fertilizer.

Are they developing a 'donut' look...that is dying out in the center? It's a common problem with Miscanthus, although 3 years is pretty early for it to happen.

Do you have Voles or other burrowing rodents in the pool area. Have ground mice dug nests in the plants' crowns?

Are you in a part of the country which experiences infestations of Miscanthus Mealybug? If so, it will spread to other plants in a group, while not spreading to plants in distant areas of the yard.

I have 3 in a row that are covering the pool filter system. Two of them are separating from the middle...drooping almost laying on the ground. One is still standing upright. I think it might be from the rain and not draining well.