Humidity in the air plays such a big part in the outcome of your final product, it cannot be understated. Unless you are lucky enough to live in a very dry area or you have a controlled environment, such as a room with special humidity controlled equipment, you will just have to learn how to deal with it in your area.
The humidity in our area comes and goes throughout the year and we literally have to adjust our formulation to suit different seasons. It can sneak up on you, so it is best to have a hygrometer or humidity meter in your production room. Purchasing a dehumidifier is a great idea, however they are expensive for beginners and won't even work if the size of the machine is not relevant to the size of the room.
There are other ways to try and control the humidity. For example you could keep your bath bombs covered with some non-porous plastic, such as painters plastic or even a large garbage bag.
Or you could buy enough molds so that you can leave your bath bombs inside the molds until they are dry. The down side to this is that you can't leave them in just any mold. The 2 part acrylic molds will fail because it breaks down and the bath bomb will adhere to the mold. The molds need to be made out of typical clamshell material, which is usually PVC or PET or a combination of the two. Both Brambleberry and Wholesale Supplies Plus carry these hemisphere bath bomb molds. The up side is your bath bombs are protected from the humidity in the air, the down side is that you are limited to sizes.
Another upside to using molds that you can leave the finished bath bomb in until dry would be that you can use the least amount of wetting agent to make your bath bombs. You wouldn't have to worry about them falling apart or going flat when removing them from the molds and setting them out to dry.
If you don't have any of the above solutions you can always adjust your formula. Less wetting agent, add cornstarch (or add more), or some other powdered humectant such as clay, tapioca starch, or perhaps arrowroot powder. Adding these will offset the cost of making your bath bombs and purchasing additional ingredients is a down side for the average maker.
If you have any ideas on how to control humidity in your area we would love to hear them!