I know nothing about this species, but it didn't take much googling to find out that it is known to suffer from leaf spot and leaf scorch diseases, and that it often suffers from nutritional problems (the page I read mentioned potassium, but who knows without some testing). As you mentioned, bone meal might help if you actually have a potassium deficiency. How much did you use and how did you apply it?
Are there any signs of pests (insects, borers, caterpillars, mites)?
Consider putting one of those scorched leaves in a clear plastic bag and taking it back to where you bought it and see if they have any words of wisdom. They might notice something you missed.