I think water and nitrogen are likely your problems. Your trees are getting a lot of water with that amount of rainfall.
I have 10 very old large Valencia trees as well as Washington navels, Meyer lemons, and Mineola tangelos.
Here is the regimen I follow with my trees. My soil is also clay like, but rich dark brown. Mineral content is good as these oranges are always sweet.
I fertilize with about 3lbs per tree of Yara 15-15-15.
On the smaller trees I use about 1lb.
It’s the nitrogen you want.
Citrus typically likes deep watering with a dry period in between. Depending on rainfall here in socal I irrigate 6-8 times a year max.