Imessage problem/bug 3G<-->wifi

Hi, hope someone could help with this "strange" issue.

I'm a new Iphone 6 user, wow what a great phone.

But I have a light problem with Imessages :

Each time my phone switches from 3G to Wifi and Wifi to 3G, the Imessage I want to send to a friend is not delivered.

If I try to send it again, same result, it's instantaneously not delivered (with the red exclamation point)

My temporary solution :

I copy the Imessage, I delete the Imessage, I paste the Imessage, I send it and then it works, the Imessage is delivered.

And then I can send a thousand if I want, there's no problem once the phone is settled on wifi or 3G.

But each time I'm leaving home or switching from 3G to Wifi or vice versa, I have to do this (copy-delete-past the Imessage) if I want to start or continue a conversation.

Strange isn't it ?

I have tried resetting network settings after disabling Imessage and then reactivating, same problem.

Is there a way to fix this ? Or is this an issue from my network/phone line provider ?

Many thanks in advance for any idea 🙂