Not receiving some iMessages

Hi all. I am not receiving some iMessages, and I don't know where they have disappeared to. Sometimes I will receive a message on my mac, but not on my phone, and sometimes I won't receive a message at all. I know this, because I will text my girlfriend back hours later asking why she hasn't responded, and she will send me a screenshot of our conversation, which very clearly shows a blue message from her phone with the "delivered" indicator. There doesn't seem to be any rhyme or reason to which messages I get, and which I don't. How do I solve this problem? Thanks.