Need a bit more info before giving an answer, so 1. how long have you had the plant, 2. In a pot or in the ground 3. if in a pot, what kind of compost did you use and 4, the yellowing - is it sort of mottled or patchy or between the veins, or just the odd few leaves going completely yellow all over.

I'm guessing it's just a few of the older leaves, which is normal for this time of year.

I had the same problem with mine that had been in a pot for a few years. I was advised to re-pot and have not lost a single leaf since I did that....yes and don't forget the Ericaceous soil.