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- German Shepherd CGC, TC
Midnite-German Shepherd CGC, TC
Brennan- Golden Retriever CGC