When the outer layer of a star, comet or any other mass of dampened composition dries in a way which traps moisture within the still wet core, the star, comet or other mass is said to be 'driven in'. While the mass will eventually dry, it is likely to take several months.
While the phenomenon is best known in compositions with high charcoal contents, they do not exclusively exhibit this problem. Another case would be Shellac bound compositions with a relatively high percentage of this resin. Another example is a NC-bound prime applied to the surface of a damp water-bound star.
Avoiding a composition mass becoming driven in often requires drying it in a cool, dark space, which may be contradictory to the instincts of new firework builders.