I Have a Sick Dragon

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My Dragon tree (Dracaena Marginata) has not been looking too well recently. Brown patches have started to appear on some of the leaves. I have consulted many house plant books and even discussed the issue on other web forums. I still have not found an exact answer to the problem. All I know is that it has nothing to do with insect infestation. Does anyone have any ideas on what might be the cause?