Need Help With my Oncidium :(

[Npsen]

I bought a very nice Oncidium in Southern California, While bringing it home (8 hour car ride) It must have been gotten a little stressed, especially because of temperature changes, It lost a few leaves (which I expected) this was in November.

Since then it has lost all blooms, all four stems, most of it's leaves it is just doing terribly nothing I do seems to work :S. The leaves feel like paper to the touch, the pseudobulbs were severely wrinkled when I brought it home, but now they are thin and non- existent also.

It was originally planted in what seemed to be moss, really fine wood chips, when I took it out it was a mess :S. Although the plant is withering away, the room system seems fine, no rot which kinda surprised me, I thought it would be rotten.

I repotted this recently, in a mix of medium wood chips, sponge rock and charcoal. I water all my orchids on a weekly basis ( except the ones that are planted in moss). I have it in a South window, that get bright sunlight from about 8 am - 1 or 2 pm when it gets a filtered light (there is a Vine around the window that filters some of the light.)

What am I doing wrong here ? I feel like..I'm watching one of my orchids just wither away, and obviously it is a fighter. The Nursery I bought it from has ignored me, and said It probably has an infection/pest problem with no further explanation !

Any help would be greatly appreciated! This is my most troubling orchid so far :S