Sorry to hear this Hawfinch, I don’t think its nitrogen if the veins are dark, more like manganese deficiency perhaps. That would be apparent on other plots/gardens in your area, have a look around. How much rain did you have in May? Perhaps if your underlying soil is sandy, that may be a factor as sandy soils seem to hold nutrients less well, although your compost should counter that.
Without more info its hard to be sure but I hope these pointers help.
