Maples, darkwood, hopseed, and redwoods can not be bonemealed. Maples also spawn with a sunflower instead of the wood itself sometimes.

The maple sapling requirement doesn't seem to be mentioned anywhere aside from this obscure forum post. Can you add tooltips to the saplings explaining their growth requirements?

Let me explain this:

Maples can only grow above Y 65 level.
Darkwood can only grow on netherrack, soulsand or tainted soil.
Redwood needs 49 saplings to grow, in 7x7 square.

Not sure about Hopseeds or the sunflower thing though.