I spaced each sapling out with 2 blocks in between each sapling, like so:
Each block of stone represents a block of glowstone, and the sapling is on grass that is at the same level as the glowstone. There is no ceiling so all trees have direct access to the sunlight. I then let them grow for a real life day. I came back to see that about half of them had grown, and others still had direct sunlight (as well as the glowstone light and no block obstructing it) but weren't growing. I then used 8 stacks of 64 bone meal on one such sapling with no luck. I repeated that on another sapling. Is this a bug or am I missing something?