Trichopilia marginata - bad culture

Good morning,
My trichopilia marginata needs your help. The old leaves turned yellow some months ago, and the new ones developed black spots.
I wish to know what I am doing wrong and how I can improve my cultural practice.
Apart from the fact that I hate to see my plants suffering, I just love trichopilias. I wish to be able to grow them, too!
Thank you,
Dana

 

Hello,
Could the yellowing leaves be a sign of insufficient N?
Could insufficient N cause black spots, too?
Any advises are more than welcomed. I really wish to be able to understand my plants better.
Thank you,
Dana