Not receiving all my messages on iPad 4

I'm sorry if this question has been asked and answered before, but can't seen to figure out the cause of why I receive some messages and others either never show up or show up days or weeks after they had been sent. This has happened several times in the last few months.
Everything is up to date. 7.1.2

Thanks for the info. Maybe with a bit of luck, when 8 comes out this problem will be fixed.