Moisture issues can be caused by the environment, your product, and even the dye itself. If water is present in the environment, it can be absorbed and retained in your product.
This happens more frequently with products like cotton. The fibers will absorb the water, and this may cause the dye to bleed. Moisture may also be absorbed into the pad of your shirt or sticker. The dye will then transfer onto different materials, and the result is very bad print quality. The best way to avoid moisture issues is to make sure the environment where you will print is dry.