Theory and rules are not enough to create good looking slides. It is a bit like interior design. Architects can use nice materials, pick matching colors, and still, somehow the overall design does not look good. And you cannot exactly pinpoint why this is.
I often have these moments where I am starting over, trying again, do something different, put a design away to give it a few extra days, because it just does not feel right, despite that I used good fonts, matching colors, the right proportions. So why is it not good? I would not be able to tell you.
This is not a problem if you allow enough time for the design process.