In "Conversations with Dead People", Dawn spends the episode fighting a poltergeist that:
was preventing Joyce from speaking to her. This information doesn't seem to be that helpful, and arguably made matters worse.
However, if it wasn't the above, and was in fact:
The First, trying to divide the Scooby gang
then who was the poltergeist trying to stop them talking?
The commentary for this episode is somewhat helpful, as far as I could interpret from what they were saying - there is the possibility that in fact the first is creating this idea of two presences in order to trick Dawn into seeing her mother as a truly good and real spirit, who is there to help her and protect her. Had her mother just randomly shown up, Dawn may have treated her presence with some skepticism. And the fact that she has to face this incredible challenge in order to see Joyce, makes Dawn understand why her mother has not visited her previously.
