Why does some wood float but some sinks

Not all woods have the same density. If it is more dense than water, then it will sink.

The most common type of wood that sinks is ironwood and there are several variations of it.

Water has a specific gravity of 1 which means that all other things are compared to water for their density. Certain types of ironwood can have a specific gravity up to 1.5.

Fresher wood (green) is much heavier than dried older wood