I just this weekend took a workshop with Martin Schmalenberg with yews dug from a defunct nursery 18 to 24 months ago. They were in turface, shallow grow pots about 18" in diameter. The foliage looked great on all 15 of the workshop trees in terms of visual health and being nice dark green. Martin said that yews are very happy with heavy feeds while in pots, and make sure that they are getting moisture to the root zone. Turface, as many of us know, may well have areas of either no water or too much water that never gets to the roots. I would suggest making sure your tree is fully hydrated by submerging the pot for 15-30 minutes, then being sure to adequately soak the soil, and feeding well and often.
I collected one from the same nursery about 4 years ago. I bare-rooted it and put it in pure turface. It basically just survived. 2 years later (spring 2014) I repotted (into the same wooden box) in a pumice/akadama mix, and this year has produced copious amounts of new growth and it has also shown new growth closer to the trunk. I also feed with holly tone in tea bags on the advice of Jack Sustic, who uses that fert on conifers in the National Arb. Bonsai Museum. Between the soil change last year and the fert this year, my yew looks like it is in the picture of health.