E-mail is not the most reliable thing.
Their server can be slow. Your serve can be slow. The mail may arrive in junk.
If you use e-mail on phone, maybe you should check if the mail sync is enabled (you receive mails at all).
If you've checked the junk, I don't see what can you do, maybe seek for support, although last time it was awfully slow.
However, I can tell you that today a mail that was sent by a service (not Bittrex) at about 11 has arrived to me some 2-3 hours later.
Last week, I've clicked at a service to send me some mail and nothing happened. Some minutes later I tried again and then I've received the 2 e-mails.
So yeah... patience and good luck!