The flagging makes me think of more than just drought stress. The next thing I would look for is seiridium canker. This is normally a side effect of drought conditions.
That's honestly the best approach. Treating them is an expensive, never ending process. Unless you just have to have them and can afford it I would recommend removal
Mmmm, don't give up just yet. I've seen this with Cypress and Redwood. (Been in a drought for a while, not now though.) And I've seen them recover with some help/water during the summer months.