Hi Letitia and welcome to GoY. Could you add a photo of the black/brown spots to your question please.

I think that perhaps a north facing aspect would not help a Garrya to thrive. I understand it needs sun or just partial shade. That might not, of course, explain the problem of the spots.

Spray with a fungicide if its fungal leaf spot - if its bacterial leaf spot, there's no treatment for that. A photograph could be useful though..