You need to add space around the trees for them to grow.
I see you are using oak saplings so you are trying to grow an oak tree.
There are several types of oak trees, including small oaks, large oaks, balloon oaks and swamp oaks. I will be finding the space needed for a small oak as they use the least space.
After trying to grow the sapling in multiple conditions I have found that the smallest space for an oak tree to spawn is:
5 blocks wide x 5 blocks long x 6 blocks high
Note: The actual tree only took up 5 x 5 x 5 blocks of space, but wouldn't grow without the extra block of air above it.
There are several types of trees in Minecraft and every tree has different growing conditions. I recommend looking at this page on the wiki for information on the love and care needed for each tree.