Thanks for all the ideas everyone!

Tulip - we do all the things you suggest. She gets two LONG walks per day (all the dogs do, but Sara gets longer ones as the energetic puppy) plus focused play time in the house and a 'quick pee' at lunch time. Sara also gets an additional walk at lunch, if she didn't poo in the morning. All three dogs eat at the same time, twice a day, every day. They are all very good eaters, so we rarely have any issue there.

I can have Sara with me, walking, for 1.5 hours and she doesn't poo.

We'll keep working with her though. I think the key is our ability to maintain our patience!