Yellowing Zygo family leaves

[Syrinth]

Hello everyone, I'm hoping you all can help me! I hav three zygo-family orchids, zygopetalum australia advance, propetalum mathina elizabeth, and zygonisia snow birds.

For the past, oh say month now, they've had leaves one by one turning yellow. First a single stripe appears in the middle of the leaf from tip to bottom, then the line spreads and gets thicker until the whole leaf is yellow and drops off.

Only my zygo family orchids are effected by this, only older leaves are yellowing, the leaves from the more recent growths appear to be fine.

My thoughts are issues with sunlight, fertilizer, or aging.

Sunlight - They're in a South facing window, about a foot away from the glass. The window has a blind on it that cuts out about half of the light I'd say.

Fertilizer - Might be giving them a bit too little fertilizer. Looking at my notes, I've been maybe doing it about once every two weeks because I wasn't paying attention.

Age - Although it's only the older leaves that have been dropping, it's been a lot of them, and from different pseudobulbs. I find it suspect that they'd all be dropping like this if it was an aging issue.

I'm hoping it's just a fertilizer issue. That makes the most sense to me, but I'm worried because I've been trying to fix it by ensuring they have fertilizer *realized something was WRONG last week* Maybe I haven't given it enough time to fix itself?