Trouble with leaves that are not really green? Cabbages turning slightly yellow?
It is likely your plants are suffering from lack of trace elements. Treat with Epsom Salts or Calcified Seaweed - available from the Trading Hut cheaper than anywhere else - other than perhaps the 'pound shops'!
*See also last week's hint - June Hint #3 - Clubroot
You can also:-
'Feed your herbs seaweed water to enhance flavour and give trace nutrients.'