iMessages say delivered but are not showing up on recipients phone

Hi, i have the following problem:

I am sending a text to my girlfriend and Imessages tells me the text was delivered. Yet they apparently dont show up on my gfs phone because she is on her phone writing me, yet the status doesnt change to read but stays on delivered, sometimes for minutes, before the texts show up on her phone. Sometimes it even takes longer and i have to send her another text to get the previous one delivered.

I mean, if the text was delivered, theres no way she could text me without the status changing to read so the messages are somewhere, but not on her phone yet.

Is it maybe because I'm on iOS 8 and she on iOS 6?

Or what other problems could it be? I asked her, shes not in an area where she has bad cell phone reception and I'm on wifi.

Its really frustrating and I have no idea how to fix this or what the problem might be.

I took some screenshots to show it:

[example #1](http://i.imgur.com/qcXkoM8.jpg)

[example #2](http://i.imgur.com/dowOu9r.jpg)

[example #3](http://i.imgur.com/TEvtxhm.jpg)