It's so sad when these lovely leaves turn yellow. It's usually down to those pesky spider mites! They can stick around for over two weeks, so getting rid of them is essential. I'd recommend cleaning the leaves with isopropyl alcohol, warm water with dish soap, and neem oil. Repeat every two weeks. You can also use pyrethrum or other insecticides/miticides if you'd like, but if you'd rather not, just cut them off. If you feel sorry for them, treat the affected leaves and others that haven't been treated because they could also have the potential to be contagious.