i use a recipe from teachsoap.com i love it ,simple ingredients&they smell great ,mine also have flat bottoms,but i think if you left them in mold for a while they would firm up ¬ be flat hope this helps
I think the trick to a good bath bomb is adding a little oil or butter. I have not had luck any recipe that did not call for them, they were either too dry and crumbly or got all warty because of the amount of which hazel that I needed to get the right molding consistency.