Gravatars don’t always show up?

My wordpress is hosted on my own server, and I’m trying to get gravatars to work with it. They show up for my email just fine, but it doesn’t work for all users. For example, I have a registered user whose account name is the same as the first part of their email (for example, username bob, email bob@gmail, although I’m not sure if that’s relevant), but gravatars refuse to show for this person’s name. However, when the user signs out of their account and attempts to post with a different username and the same email, the gravatar shows up just fine. How would I resolve this problem?