Switching the food frequently, or too quickly reaks serious HAVOC on the GI system. Even too many bones in conjunction with the regular food will cause upset. Their little tummies are sensitive.
I would try a food and give it at least two weeks for their bodies to adjust. Also, they may be allergic, or sensitive so try and find one with salmon as the main product instead of beef, lamb, or chicken. I know Purina ONE makes one for sensitive stomachs that is dominantly Salmon.
Good luck.
Till then, maybe keep them on tile or outside till the food is expelled.
