Apple Watch 4 Apps not loading from IPhone 6

Hi I have a Apple Watch 4 with a iPhone 6. A number of Apps, such as Strava, Outlook, Mapmyrun will not install onto the watch.

What happens is the App goes through the download / install process. I can see that apps starting to load onto the watch. however right at the end the app doesn’t load.

i have followed the advice I have seen online including a full factory reset of both the phone and the watch. Both are now using the latest software.

Curiously this seems to be an iPhone 6 issue as I have the same issue when I pair this with my wife’s old iPhone 6, but it isn’t an issue when I pair the phone with her newer iPhone 8!!!

Has anyone a solution for this?