That has happen to me when I first started make clay centers.
What I learned is to add Liquid Sculpty before baking to seal everything together and then add the glaze after they come out and cool down.
Your people who are making them probably do not know about this, it was a learning lesson for me.
Just ask the people who you are buying them from to please do this.
Thats all i can advise
I made many bows just to have the center fall apart