Birch saplings won't grow through birch leaves

I'm trying to get a birch farm started in my game but the leaves from the grown trees seem to be preventing the saplings from growing. I have 2 blocks of space between saplings and at least one block of empty space surrounding each one at ground level, but the saplings between trees won't grow unless I remove the leaf blocks above them.

I used to have a spruce farm in the same spot with the same layout and never had an issue; the problem seems to be exclusive to birch.