iMessage isn't working when using wifi

I recently started using group chat via iMessage for school work, but the messages can't seem to get through both ways when I'm using wifi. None are coming in from the group members and none of my messages are delivering. But when I turned off my wifi and switch to cellular data the messages are coming in just fine. What's weirder is that, I can receive messages from private iMessages so I know it's not my wifi that's going crazy at that time. Anybody care to provide me some solutions?