Why can't I receive messages from other iphones?

I have an iphone 3gs and I noticed that I won't receive messages from other iphones until a while later, I only just got one from 3 days ago. Can someone tell me how to sort this out I've reset my phone like 10 times and its still not receiving messages.

Is it just iPhones? Is it some issue related to iMessage? (if the other iPhones are attempting to send messages via iMessage and you do not have your internet enabled it may cause the messages to be backlogged, I should imagine).