I had a similar issue and it drove me nuts. I started feeding them less and no more problems since, I was over feeding them. It was a simple fix, I hope you can figure it out.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD