Why won't my saplings grow?

[Plume]

I can't figure out why these four saplings will not grow! I've had bushes grow in these locations before. Does anyone know what could be happening?

 

[BluebellLight]

You can't have more than 12 bushes in a row

 

[CR33P]

i think this is the problem

 

[Krissi2197]

You can't have more than 12 bushes/trees in a row. :c

 

[Plume]

Ah, that makes a lot of sense! Thanks. c:

 

[Amykins]

Yeah, I usually stagger mine two to one for this reason. It still looks good, and no risk of gaps ^^