Why isn't iMessage working?

My iMessage delays replies. My friend will send me a message, and sometimes it won't show until 3-4 hours later. When I finally do get it, it says the message was sent a few minutes before the notification appears. For example, my friend sent a message at 7am, but it did not show in the app until 11:30 that morning. The time under the message said it was 11:20. The app does not always do this. It's so unpredictable because I'll be talking with him, and then it will just stop working. Is this normal? Is there anyway to fix or reset it? I'm sorry if there is an obvious solution, but I'm the dumbest person when it comes to technology. Thanks for the advice.