Annoying problem with texting.

Hi

My phone ( iPhone 5s ) often has a problem texting.

I'll send a text and it wants to go out as an iMessage ( blue field ) - but will fail.
I then have to check whether it sent, and when I find out it hasn't the manually opt to send it as a normal text ( green field ). This works fine.

I though that it would automatically choose which type to send - i.e. if iMassage fails then it would automatically send as normal text.

Any ideas to fix this?

Thanks all