Thanks to all.

The humidity is certainly lower than 60% in our summer. Another possible factor is the calcium level. I have compared throughout our few tests and re read an old article i had saved.

It suggest calcium and my records indicate i was low in summer when the problem occurred. This season i will be acting faster with the sap test and erring on the higher side with both the potassium and calcium. the humidity…well thats one ill just have to take on the chin, without any overheads in operation here.