Dark Oak Trees are not growing even though all requirements are met
Solution 1:
To grow dark oak trees, you need 4 saplings in a 2*2 box. A single dark oak sapling will not grow on its own, regardless of how long you wait or how much bone meal you use on it.
In this image, the saplings on the left will eventually grow to form one dark oak tree. The sapling on the right will never grow.