If bonemeal works, maybe its just the height limitation. The wiki says:
Both birch and spruce trees require 9 blocks of vertical space above the sapling to grow regularly (10 is more efficient for growth). Both spruce and birch grow with less space, but not as regularly. Like other saplings, they must receive light level 8 or better to grow.
So maybe a tree will grow at six blocks, but its just very slow.