In my experience if the spindle and board are too soft they tend to reduce each other to powder before getting hot enough to produce good coals. Slightly harder wood seems to work better. On the other end too hard a wood can build up a lot of heat but fail to drop a coal.

The wood, of course, is best very dry. Slightly green or damp wood can make things much more difficult. You have to generate enough heat to drive off the moisture before you can start to get near ignition temperatures.

Irony is that drying the spindle and board over a fire makes them work better. Kind of like needing a flashlight to change batteries or downloading a better driver to get the PC to run. Bootstrapping has always been an issue. I knew a Boyscout leader who would use a blow-dryer to prep his bow-drill setup before demonstrations in a damp climate.