Email providers in general, and GMail in particular, is not as sophisticated as we like to believe. There have been so many instances where people I have been talking to for several years get randomly thrown into the spam folder.
It goes without saying that some of my outreaches get thrown there too by default.
So, as the sender, I don't really know if you saw the message and didn't want it, or if you were too busy to reply, or that you did not receive my message in the first place.
There are, of course, many email tracking tools available. I do not use them though.
Of course it doesn't work for non organised individuals and people who succumb under pressure.