A particular balance must be achieved when making homemade bath bombs, and it's this equilibrium that often causes people the most trouble at home. According to Daisy's Beauty Jewels, bath bombs can easily become too dry or too wet. If they're too wet, they won't hold their shape, and if they're too dry, they crack and crumble and don't form their ball shape. Your mixture should be the texture of kinetic sand before you mold it, meaning that it's slightly crumbly, but still holds its shape when clumped together.
The order in which you add your ingredients matters too. Citric acid, for example, should be added after your other liquid ingredients (via The Homestead Challenge). Adding it too soon can cause chemical reactions that will make your mixture expand, meaning you'll never get actual bath bombs. This also means that your bomb, if it does set, won't fizz much in the bath because the chemical reaction between the acid and the baking soda already played out in the mold or your mixing bowl.
Similarly, you want to research recipes before you use ingredients such as coconut oil. Coconut oil is solid at room temperature, but it melts easily, so if you use it as a structural element, you might be in trouble if your house warms up throughout the day.
Still, though, even if you run into mistakes and your end product isn't a perfect bomb shape, you can still use what you've made. You can try to fix it by letting it sit overnight (drying it out), or you can use it as a bath dust, which will still dissolve in your water (via The Soap Guy). Since there's a reward no matter what, you should try making them for yourself!
