One possibility for lobby stools is that it is going through too fast. I had that with my 10 yr greyhound - it was the biscuit not agreeing whereas the mixer had been fine.

As one person has already suggested; fish is ideal easily digestible - i'd try chappie or a tin of pilchards and see how that goes. Again the trouble with kibble is you are not quite sure what is in there. Could be a biscuit allergy?

Use some rice to bulk it up before you try mixer. See how that settles.

I've had dogs all my life and Chappie is i reckon the best commercial dog food. Alternatively go to a butcher and see if they do a pet mince - they'll tell you exactly what they put in it.

Fish and rice are good stomach settlers for dogs. Once their tummy is settled you can introduce and gradually change to what you want. For health reasons i believe in plain meat, little veg and mixer biscuit. It doesn't cost more than commercial dry foods.

Vets are not nutritionists - they'll recommend some dry food. Dry food contains ethoxyquins (HDT, HDA) these preservatives are responsible for a lot of health problems. google ethoxyquin
The Problem with Ethoxyquin