One side of a leaf turning yellow on phal

[Fuzzy]

Hello,

My lovely Kaleidoscope Golden Treasure phal is not happy. One of the leaves is turning yellow on one side, starting at the crown. What could be causing this and how can I make sure this does not spread further? Am I going to lose this leaf? I hope not, the phal has only 4 leaves and they are all pretty weak. Another question - is there any way to encourage a phal to put out a new leaf? This phal has been in bloom since I got it in late March. Could this be the reason the leaves are not happy?

On a more positive note, the roots of this phal are looking good as far as I can see. It's even growing a new one I posted earlier about (the pic of the new root is attached here as well).