
I have known a few people find they can't keep weight on with burns, so I suppose it depends on the dog.
To the OP have your tried a different puppy food on your bitch.
when I tried the Pedigree professional on my girl (I had won stakes places and needed to use up the prize vouchers) it went Right through my girl.
Normally they are best on the food that keeps them firm when not in whelp or rearing, but say the puppy version.
Milk, even puppy milk (unless it is made from goats milk) will often go through bitches.
If my bitch was not tolerating the puppy food, then I would as a last resort put her back on her maintenance food, but increase it to four meals a day, giving her the amount she would normally eat at each sitting.
As you ahve had problems keeping weight on her in the past it could be that she doesn't absorb nutreints from her food well. Which food if any has she ben really firm and done well on, that is what I woudl be feedign at thsi stressful time.
In my experience Beta isn't as digestible as some other foods.
My own have kept nice and firm on Arden grange Prestige. I have been able to give any amount of this without looseness. did get a bit of looseness on the other hand with James Wellbelloved.
With pups I have always used several brands of puppy food mixed together so that the new owners could choose what they liked from these and the pups would still get a food they were used to, once the supply from me had run out.