There could be a couple things going on here that could be part of the reasons why it is doing poorly.
First the plant appears to be planted extremely deep. While deep plantings themselves don't kill hostas, if the soil that deep is compacted then the plants become oxygen starved and struggle. Also water has a harder time getting that deep and they become more drought stressed. In time a hosta will establish a new crown higher up, but this plant hasn't had a chance to really start doing that.
The other issue could have been with some kind of crown rot, possibly Fusarium rot which favors dry conditions and then gets into the crown itself. Often some eyes are able to fight it off but you will see brown leaf edges in mid to late summer indicating that the Fusarium is damaging the roots and crown. Southern Blight, on the other hand, only affects the surface of the plant, basically rotting stems, and has absolutely nothing to do with the plant parts under the ground, so that isn't an issue here.
So taking the second item into account I think that the roots were drying out too much. Again, that could be a depth issue that wasn't allowing water to get that deep. Make sure you aren't planting things this deep or mulching this deep. Larger hostas light Olive Bailey Langdon can be about 1" to even 2" deep but don't go deeper. And keep the mulch depth as part of the overall depth and don't add more than an inch right around the hostas. For most miniature hostas you don't want more than about 1/4" deep.
I hope that helps a bit, and good luck with your five plants.