I know what you mean, I've had a few I've popped a acknowledging hand up only to have them ignore you when you know they saw you. Yet others have enthusiastically wave I thought they would go off the road.

I had one guy parked right next to me at a gas station where I was getting coffee and he was too. I said hi and he never even nodded. It is what it is and if they don't want to reciprocate, oh well.