Hi Biccy. Could be a number of things. Leaf miner is a possibility, or a touch of heavy frost. Unlikely I suppose as the others are OK. So I would check whether the soil has the right PH balance but more importantly is the Azelea perhaps "root bound"? I think that I would dig down into the pot and check whether the soil is either very wet or very dry. If it is particularly wet, you have a drainage problem with the post. If it is very dry, and the root ball is a tight mass you may need to think about re potting it. If it has been in there for any length of time it may be ready for a bigger plot.