There are several reasons enamels crack. The most common reason enamels crack is there is not enough counter enamel on the jewel. Counter enamel is enamel on the back side of the jewel. Your base metal expands as well as the enamel when heated, and contracts when cooled. Without enamel on both sides of your base metal one side is going to expand more causing the opposite side to crack. If enamel is placed evenly on both sides of the metal you can eliminate this cracking. Sometimes this happens as it is cooling, when this happens there is very little enamel on the back. But I have seen the enamel crack years later when the enamel was not even on both sides of the base metal.
In some cases you can also dome a piece of metal and use less counter enamel as in a bead. But if you plan to put a thick layer of enamel on the front of the metal you then need more counter to avoid cracking.
Another reason enamels crack is if you use a hard firing enamel next to a low firing enamel. Hard firing enamels take longer to fire, and low firing enamels take less time to fire. Great reason the make test plates and see when each enamel melts first when they are all fired at the same time on the same plate. You can see on my test plate dot #4 is still grainy while the others are smooth. So #4 is a harder firing enamel and if placed next to #5 and you fired it long enough that both enamels were smooth in time the jewel will crack.
Cracking takes place in leaded enamels as well as unleaded enamels. Unleaded enamels are harder firing enamels than leaded enamels so if you combine them on the same jewel you can experience cracking. If you need to use them together use the unleaded first then layers of leaded to avoid cracking.