Usually happens if the receiving iPhone is not using an Internet connection, then your iPhone sends an SMS text messages instead of an iMessage.

iMessage only works end-to-end if both iPhones have a network connection - either using the network operator mobile data or by using Wi-Fi.

https://support.apple.com/en-gb/105087

This Apple article should help, @Snipepie67. 

Thanks very much

Hi @Snipepie67,

Welcome to the Community!

It looks like @andewhite has answered your query.

If you require further assistance, please let us know.

Kash