My deer ferns are dying...

Hi,

I bought 3 deer ferns last fall and planted them in an admittedly FAR too carbon-rich area (lots of woody mulch in the soil). But, after a while, I noticed that one was starting to turn a light green & then brown, so I removed all 3 & potted them with some bagged outdoor soil + poor clay soil just to let them recover a bit. That first fern seems to have all but died off, despite my rescue attempt, but the other 2 looked like they were going to be ok. However, this past week I noticed that they were both starting to turn a light green too and so now I'm really nervous.
Is there anything I can do to save them??

'Indicator Plants of Coastal BC' says that the deer fern occurs "on fresh to very moist, nitrogen-poor soils... Grows best on well decomposed (greasy) organic materials; on nutrient-rich soils, confined to decaying coniferous wood. ... An oxylophytic species characteristic of Mor humus forms." So that tells me that they supposedly LIKE fairly (but not overly) carbon-rich/nitrogen-poor soils, particularly if there's only a ~5cm layer of fine primarily-woody organic matter decaying on top of mineral soils. Right??
So wouldn't these ferns do well in some nice store-bought soil (+ woody/leafy mulch on top) that's layered on top of my dense clay soil? Or is there something else that I should do/use instead? And, perhaps more importantly, why aren't they reasonably happy in the unoffensive mix that's in their pots right now? Or have I got this all wrong and my ferns are affected by something else entirely?? Ahhhhhh!!!!

I'm so confused. How do I save my poor deer ferns?