Phal gigantea with yellowing leaves

[fotofashion]

My P. gigantea is losing its leaves one by one. It dropped one leaf last week and the next oldest one is turning yellow (really more a gold color) from the tip. This is not a seedling plant either. The leaves are a good 12" long. I paid a lot for this plant and do not want to lose it. The first sign of problems manifested itself in the spring when a new leaf started growing. It got about 1 1/2" long and then started drying out from the tip. It has not developed any further. The plant is mounted on a bark slab, is hung where it gets good light and fertilized weekly/weakly. Help!
Beverly A.