Yellow cat. leaves on older growth

[Orchidnut]

Hello,
I have a question I grow in a greenhouse. I have 50 % shade cloth on roof and sides of it. I noticed that my older growth on most of my cats. are quite yellow the newer growth this year looks good. I also have a few phals that are quite yellow looking. I was using a organic fertilizer but have recently switched back to what I used to use. I have had the greenhouse up and running for 2 years now and l never noticed the leaves being so yellow before I did not change anything shade wise. I have used some horticulture oil to control pests before but nothing recently... any ideas why so much yellow on my plants. They are growing fine and flowering but older growth is yellower than I would like it to be.
Thanks
B