iMessage Appears on one device only

Hi - I've a strange issue that's only started happening since upgrading to IOS 8.x on iPhone 5 and Yosemite on my 2013 macbook air.

If I am in an area of poor service, IMessages are not delivered to my iPhone. I'd expect them to arrive once I have better service however they never show up.

Instead, when I next start to use my macbook, the iMessages appear there in Messages. Yet they never appear on the phone, ever.

Both iPhone and macbook are configured to use my phone number to send messages and receive messages on my phone number and two email addresses (i.e. my apple id and an iCloud email address). And my phone has "Text message forwarding" enabled to my macbook.

The concern is that I will never get the message unless I use my macbook.

I've tried to recreate the issue but it appears intermittent.

Any help will be greatly appreciated.