What do the ones still left in the shop/source acting like? As said earlier what are they feeding them?
If there were ph differences at the shop/source to your place, or if the yellows had paraisites, nitrites,ammonia ..... then the fish will be breathing heavy.
From what you say they are not, which leads to internal problem, bacterial infection & the like. Which leads me back to the source.
When I bought my yellows(got about 20 from 4 places)
2 of them I noticed after a couple of days acted different to the others, not as active, anyway both died. (I jacked up UV Sterilser to tank for security) All others were/are fine. My thought was that the "2" were very similar & probably ones I got from either source 3 or 4. From which I only got 2 yellows from each of. On returning to just have a look & see, at one source there was a dead yellow & a couple not looking good.(They seemed fine 5 days earlier) But another important point, they were on a system & they had problems in other tanks. A reminder to myself to check properly next time. To me no point having them on,(I got a shrug of shoulders when i noted the deaths) I wouldn't want a fish again from them.
I am sad to here your loss, if I was there I would donate a couple of mine, yellows are such a lovely fish. Anyone there have good yellows?
Frenchy