Some texts send, some don't. Can receive some texts, but not all.

I have an iPhone 4. I recently talked to a friend that was wondering why I haven't responded to several of his texts. I get some of his texts, but not all of them. Some of the replies I have sent him have not made it to him. I have good reason to believe that his has happened with a few other friends as well. Some are iPhone users, others are not. Here's the really weird thing - there are other people, like my sister, that seem to get every text I send, while I also seem to get every text they send. Does anyone have a solution for this? One of the people I am having the problem with is a person I do NOT want to miss messages from.

I am on an iPhone 4 on AT&T. I recently upgraded to ios5, but I suspect this had been a minor problem before the upgrade. It has become a major problem now. I tried turning the pone off and back on, deleting tons of text messages, and turning off iMessage. Nothing has worked. Please help. Thanks in advance.