We have four very mature large camillias in our front garden. They have grown happily for over 100 years and each has a different coloured flower. At the moment all of them have buds and one or two of them are now in flower which is normal. However the smallest isn't well - part of the plant has died and some of the leaves are turning yellow - although some parts of the plant are healthy and producing buds. Can anyone advise us please.