Did a search, but could not find the answer, and in case I missed it...

Ok, so when you do a booking, sometimes the option comes up to use Avios to part pay towards your fare. like 6 different options.

And sometimes it does not show?

Do any of you know why or under what circumstances does it show?

On our flight to EWR it shows, but not to Tokyo?

all with BA metal, booking in BA.co.uk website

thanks.