As someone who has a constant tide of emails, I can vouch that it is not always easy to keep on top of them all.
For example, Outlook has this habit of highlighting the next unread email when you open it. After a short period it marks it read, even if you haven't actually read it. It's very easy for emails to get overlooked this way as I know to my cost.
Also, sometimes you read an email and can't deal with it straight away. If you make a mistake in marking it (flagging, categorising or just marking it unread again) you probably won't find it again and deal with it.
It isn't unusual for emails to be classed as spam as well. I've had emails from my wife and even from me to one of my own email accounts diverted to 'junk'.
It's also possible people are on holiday etc. Even in the 20th century some people don't reply to work-related emails in their personal time and also prefer not to put up 'I am on holiday' for obvious security reasons!
Not excuses, but reasons why sometimes emails get missed despite people's best intentions. I try to look through for emails I've missed at the end of each week, and sometimes discover ones that are even older.
My advice is never be too quick to judge if an email goes unanswered; let's face it few people will deliberately ignore a genuine message. Just send a polite reminder with a copy of the original.
Neil