Find My is suddenly broken on one device only

I use Find My to help keep in touch with family members. For no reason I can think of, I can now no longer follow my wife's location on my iPhone.

I can follow it on my Mac. And on my iPad. Just not on the phone - which is easily my default device.

Other family members are not effected by this.

After checking the obvious and doing lots of googling, this looked pretty comprehensive - https://www.lifewire.com/find-my-iphone-not-working-1999175 - but none of the steps outlined help with the problem.

Any help would be super welcome!

TIA,

Mike

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