
Appologies if this has occured to you already, but just an initial thought; Have you checked her teeth and ensured its not a discomfort issue as opposed to a diet preference?
How long has she been off her food? NO dog will starve itself generally and its a vicious circle to get into to chop and change inless you know what your doing (may be another underlying health issue?)
And once those possibilities have been ticked off, then Im afraid despite what comments you get,you may well just have to 'suck it and see' Dogs seem to be differing so wildly these days.You will have just as many folk with good reports as bad! In the main I think its pretty much agreed that the main criticism that comes up of Burns is that folk find dogs tend to loose wheight/ cant keep weight on with it. Your dogs may be the same they may be different! frustratingly ;)
And protien level requirement will vary as to your individual dog too.Mine for example are somewhere between Sedentary and working dogs so I use higher protein diets than for example my mother with her 'stay under the quilt all day Whippets'! ;)
HTH?