The plant sure looks healthy...You may want to take it out of the pot and look at the roots....if there are lots of dead roots, you may need some fresh potting mix. When is the last time you had some new growths? My nobiles are sending out new shoots now. Most of mine are in baskets with sphagmum moss, though some are in plastic with bark, and some in clay with sphag.
Being able to figure out what you are doing wrong is one of those things that takes experience and careful observation, and will depend on your growing conditions. It may be time to repot...but most dens. do not like to be over potted, and do best when a little pot bound. I've been using a lot more sphag in unglazed clay pots (the clay wicks away the moisture) and baskets, and I think the roots have an easier time growing through the soft moss, but then again, my plants are in a greenhouse and get lots of air circulation.
Another thing I've noticed is that orchids like a little pruning every now and then...it stimulates new growths. Most nobiles will bloom from old canes that are still green, but if you don't have new growths your not going to have that specimen plant we all strive for. If the yellowing cane is the one without leaves, you may want to cut it off at the base...with a sterile blade...I wish I could be more helpful...