They probably don't want to be bothered so they're ignoring you. It's pretty assholish unless they're constantly bombarded with reference requests even from people they barely even knew in passing. This is why it's important to make and keep some kind of contact (i.e. fake friendships) with people you work with. This is true in particular of your immediate supervisor.
If you're finding yourself with no other option, you probably want to keep a-knockin' until they begrudge a response.