Q: imsg to iPad and iPhone

Hi people, looking for a bit of help here on imsgs.

I have an iPad air (with sim) and an iPhone 6, both on EE, both logged in to the same apple ID.

Both iPad and iPhone have imsg turned on, both have 2 addresses for delivery - my email address and the phone number of the iPhone.

The problem I am having is this:

If the iPhone is off for any reason (e.g. happened when on aircraft last week) then the iPad will receive any imsg sent to me, but when the phone is turned back on, that will NOT then receive it.

I've also got an iPad 2 and iPhone 5 set up identically, but no matter which is off or on, when they're switched back on, any missed imsg's get downloaded - how can I get the air / 6 to do this too?