Stroke and coat running. What am I doing incorrectly?

I made a few mugs for some coworkers and I wanted to do some funny coffee inspired sayings on it. I used a circuit to create the template. After having my underglaze flake of my bisque one of my teachers suggested stroke and coat. I used different combinations and did three coats for the base, placed the stencil, three coats for the lettering. The inside of the cups were glazed with studio dip glaze, nothing else on the outside but stroke and coat. Fired to Cone 6 Any insight on what could have happened? I thought S&C was supposed to be stable up to cone 6?