The email was probably flagged as spam somewhere and blocked.... Put the eBay app on your phone, and set it to only give you alerts when you get messages from buyers, and when items are paid. This will give you what you need, without things that you don't. Even these alerts don't always come through. I also get emails that don't come through until a day later, however I sell enough that I see that it sold when I load orders in my shipping program. Explain to the customer what happened. As long as you communicate with them, and apologize, it usually turns out OK. What really sets people off is when there is a problem, AND they are ignored.
