imessage not getting through

I'm totally confused as to what's happening.

My friend is holidaying in the Irish Republic, I'm in the UK. This past number of days we have been trying to contact each other, we both have iPhones but we can't seem to maintain a regular contact. When we try to contact each other our iMessages do not get through, very occasionally they will.  Sometimes I've sent a message it will show blue and then after a few seconds turn green.  I'm sitting looking at an iMessage now, its 9.59, and it's marked as delivered, I sent another message at 9.08 but it's green!

My friend asked at a local phone shop and they told her the network (o2) was down but this has been going on since late Wednesday night, I can't see a network been down that long.

Any ideas of what's happening and what I might do?