Gmail's magic algorithms are notoriously unreliable. When they work they work, but when they don't, they really don't. And you won't know, because them not working means you won't get any kind of notification they don't work.

I've gotten direct responses sent to my spam. I've gotten emails, WITH ATTACHMENTS, sent to my spam from a known email address. Its a good thing I check my spam. Many (most?) don't.