I was trying to hatch a Blue Fire Dragon on DragonVale but had problems doing so. Does anyone know what my issue might be?
There are a number of combinations for the Blue Fire Dragon listed on the DragonVale Breeding Sandbox (https://www.2084.org/dragonvale/breeding-sandbox/).
While the success percentages there are only estimates, in my experience they are close to reality. So unless one of the parents is a Blue Fire (which you probably don't have), you have a 10% or less chance of getting one when using the island and 5% or less in the cave. That means that you will on the average try 10 times or more (20 in the cave) before getting one.
Remember also that these are averages. That means that some of the time one person gets one after a couple of tries, but another might not get one until the 40th try.