I agree when someone bothers to comment on a post, they should show the courtesy to upvote the post, even if they agree or not.
The explanation that comes to my mind why someone would comment without upvoting (unless it simply slipped their mind), is if they have a low voting power, but they still want to engage with other people.
sorry for the late reply. their is something on it...
No problem. Not sure if that's what happened in your case, but it used to happen to me and I was frustrated until I found the solution. People replied to my comments right away, while I saw them sometimes hours after.
It was obvious they used some kind of a tool to be notified of new replies, I just didn't know what it was.
Then I found them, it may help you too, if you don't know them yet. You have two great options, one is Steemify from @blockbrothers, the other is @ginabot which works on Discord.