Dragon tree leaves snapping off midway down

Hi there,

I'm hoping someone can help me identify an issue with an indoor Dragon Tree I have. It's generally quite healthy, with plenty of leaves growing and good colour but some of the leaves, more or less ever since I got it, get little spots on them and then snap off mid way down. It looks like something is eating them but I can't see any pests. I've sprayed it with some general stuff for pests but to no avail. Any ideas?