We all know how annoying it can be not to receive a reply to a well-intentioned email. The default is the person is probably 'busy'. Busy can mean many things, but it is just one of many non-mutually exclusive reasons for not receiving that awaited reply in timely fashion or at all. For example, the person is:
Organized, but feels that your email is low priority or irrelevant
Can’t cope with huge numbers of emails
Not looking at email as often as you think
Waiting for a reminder
It's the weekend, a holiday, or on vacation
Didn't realize that you are expecting a reply
Fears that you will just send (yet) another email
Avoids an escalating conflict or embarrassing misunderstanding
Feels somehow uneasy about content or can't come up with an adequate reply
Views your email as annoying, aggressive or inappropriate
Did receive the email, but was the unintended recipient
Did receive the email, but waiting on someone or something
Did reply, but you didn't receive the message
Didn't get the email - inadvertently sent to someone else
Didn't get the email - changed email address
Didn't get the email – stuck in sender mail-box; spammed, trashed; address
Does not want to seem too interested, or just wants to look cool
Thinks that you or your email are unimportant or irrelevant
Has ego issues, low empathy, or is just a plain asshole
Has poor eyesight
Server or Wi-Fi is down
Is dealing with health, personal or professional issues
Has retired or passed
Regardless of which of these, your expectations may just be (way) too high.