New Phal leaf limp & a bit yellow

[Helen]

Hi all,
I'm back! It's been a busy summer! I have a quick question on my phal.. My latest leaf is a bit soft (kind of falls to one side and not sitting up on it's own very well) and a bit yellow where the growth is coming out...the majority of the leaf is a nice green...I was wondering what this means? My other phals are all having new leaves too, but this is the only one that has this problem...it is in a north window and not getting as much light as the others...but phals like the shade don't they? so I don't think this is the problem...it is not overwatered, it's in a clear pot and the roots are plentiful, plump and green....it's in the original mix (looks like coconut fibre or something like that). This was a NOID I purchased back in May.
Thanks
Helen