I agree with Tony but suggest that when you are ready for a kiln, consider a small kiln with a regulator( for safety and accuracy), instead of the beehive. A kiln seems like a daunting purchase ($800 and up), but they last forever with basic care, and you can easily sell a used kiln in good condition if you decide you are not using it enough. It’s more sturdy and larger than a beehive, so you have more versatility. There is nothing wrong with a beehive, but if you t think you will continue to enamel, a kiln is a good investment.
But back to the original problem: are you using opaques on the back ? Try using dark transparent colors on the back. Blues ad greens are generally more stable. Or maybe try Thompson 1995 black.
Experiment with different enamels to see what works best with the flame. The old leaded enamels are not good. Also you may want to design to take this into account by setting it in metal, or doing other things to conceal the back. Another possibility is to grind a texture or pattern on the back with diamond bits in the flex shaft. Hope this helps. Best of luck in your enameling adventures!