Certified GKH Gardening Expert
Excessive leaf drop my indicate overfertilizing or be a sign of too much/too little water. In addition, it could be a sign of leaf gall. The following article should be of some help to you: https://www.gardeningknowhow.com/ornamental/flowers/camellia/identifying-and-fixing-problems-with-camellias.htm
Most pests that feed on camellia leaves can be treated with neem oil.