Phal Leaves Turning Yellow/Falling Off

[Royal]

Loosing the newest leaves is not a good sign. I'd separate it from the other healthy plants and hope for the best. It may be a lost cause, but it could bounce back and push out a basal keki (a new set of leaves from the bottom of the stem). This would take a while but it's worth it if the plant is especially important. I wouldn't get my hopes up though.