Slipper spots???

[kiki-do]

Hi All, One of my slippers is beginning to have one leaf turning yellow (bottom one) and the others have these brown spots, however not too many. This slipper has not blossomed in over a year. This is my problem with slippers, they only bloom the once and not again, but do produce new leaves. Anyway, I try to keep it moist, but not overly wet. I have it in medium light, not bright or direct. I fertilize every other watering. This one is in a clay pot, so I have had the clay pot sitting on pebbles with water to help keep moisture around it. Never directly in water, of course. I never mist them.
What am I doing wrong? Or right? What is causing the spots and yellowing? Thanks for any suggestions and/or ideas of what this may be.