Noticed this issue a few times on my DS. The NCF will flash between purple and green with the ball effect raising and lowering and no nanites being released. The status in the terminal is listed as 'SpoolingDown' and the block does not construct. It has more than enough resources and access to assemblers and resources to build anything it could need.

I've noticed this seems to happen most often when the NCF has a large number of construction targets available.