One person not receiving my texts (problem)

I have a Samsung S9 and have been using Textra on previous phones for many years. A couple of weeks ago, for no obvious reason, I cannot send texts to one particular person. This person DOES receive images from me, just not texts.
This person DOES receive texts from me if I use the Samsung Messages app.
I have tried everything I can find on the web, including deleting Textra and re-installing it, deleting this person from my Contacts and re-creating them - all to no avail.
I have checked their phone (iPhone 8) and I am NOT in their block-list.
I have spent hours on this and it's driving me nuts.

If anybody has any useful input I'd be grateful.

We are both in Australia and I am using AmaySIM and she is with Optus (so both Optus).