tree cannot grow, only oak

the sapling (of the picture) cannot grow, only oak can grow in this situation, I can't found help or wiki for explain that...

https://www.youtube.com/watch?v=s6nE__371LI&feature=youtu.be

 

Observer* is bogged ? oak need too much meal in other map (same redstone system) for grow (a stack of 64) O_o