When put on MOP on china don't paint it on in lines.
If you apply it in straight lines, it will fire in straight lines ...( Actually , the straight lines can be effective on things like snowbanks, etc ... and Kerri Manuel recently had the coolest piece on Facebook qwhere she applied the MOP in deliberate streaks on the rim of a plate.. like rays of the sun ... It was really neat looking).
Swirl the MOP on the china piece with a finger ( could be with finger cot) or q- tip that brings out more of the colors. Do not use a brush. But, you can apply it with a brush in swirls too.. but its the swirly pattern application that will give you the nice broken up pattern that is so pretty with mother of pearl ...
Also, the MOP should be thin, watery. Too thick MOP layer application will blister and burn off .. ( That can be reapplied ). Luster does come off fairly easily .
Why it happened?
Either it was still too wet when you fired it....most lusters you can fire wet and it doesn't hurt but MOP is a temperamental lady. One time you fire her wet and she is fine, the next time and she looks smokey, streaked or opaque.