Overwatering, underwatering, not fertilized, too much fertilizer, underpotted, salt/mineral build-up in soil, not enough light. Could be a number of things. Tell us what its environment is like. Can we see the entire plant, as well as the soil?

It is in an well lit orangery, but in a shady corner so not in direct sunlight. There has been some fluctuation in the weather recently so am wondering if it could be that- have had about a year and a half and it's only in the last few months it seems to be deteriorating. I backed off with the watering/misting for a bit to see if that had an impact but no success. I add a few drops of baby bio every couple of months. Any advice would be greatly appreciated- I really don't want to lose it and don't know what to do!

Looks like it needs to be potted up to a larger pot. Fresh soil.