No iMessage alert when receiving messages

For a while, everything ran fine — I'd get an alert on my computer when I got an iMessage from someone. But recently, the Messages app doesn't bounce when I get a message, a banner doesn't pop up, nor does the alert tone play. The only way I am able to get notifications for it is by completely quitting it, which isn't very practical to do every time I send a message and wait for a reply.

Any suggestions?