In a minecraft 1.2.3 world, I have created an underground area for growing trees. The four dirt blocks you see (two on left) all can grow trees, but with the others, they never grow, even when using bone meal.
The vertical wall is six blocks high, and they all have two clear spaces beside them. The only thing is that the one dirt block you see closest is also just outside the opening of a straight and normal stairwell. Does that affect it too?
6 blocks high isnt high enough for certain size trees to grow, i would suggest going to 8 blocks high as well giving them three blocks of space on all sides. i would also suggest making the square with the sapling on it one block higher then the floor with a torch against the higher block. trees will just refuse to grow if there is not enough space for them to grow.
trees grow a certain size randomly so for example if a tree growing 8 blocks tried to grow in your small 6 block high it wouldnt grow at all, this is why bonemeal is not working properly. i wouldnt suggest going higher then 8 blocks unless you enjoy climbing through leaves in search of random wood blocks
Try placing torches in the squares immediately adjacent to the saplings. That will ensure that they have enough light.
Then go get some bonemeal from your friendly neighborhood skeletons. Apply the bonemeal to the saplings: boom, instant tree.
When I grow underground forests, I also put torches on the trunks. That way, I know that there's no chance for a stray shadow to either slow the growth of the trees or to give a pesky zombie a chance to spawn.
From what i know, they need a lot of space so they can grow, and since they can get pretty big, even with a single sapling, you should put the sapling at least 5 blocks from an other and make the room bigger. There are only 2 blocks on a side where the leaves can appear, so make the room bigger and it will work. There are also the torches wich stop them from growing, so remove them and place other light sources, like glowstones(if you have them), or a lava block stopped by glass.