When you have a task group in orbit of an asteroid, and you issue an order for a ship with Asteroid Mining Modules to move from your shipyard to the task group, with an order of 'join', the number of mining modules listed in orbit of that asteroid (and the resulting increase in mining output) does not register until you issue an order for the entire task group to 'move' to the asteroid that it's already at.
It works fine if you tell the new ship (to be joined to the task group) to it, and then use another method to join it to the larger (eventual) task group.