IMO, ROTS could have continued with the idea of Qui-Gon only "coming through clearly" because of Anakin's pain. In AOTC we hear Qui-Gon's reaction to the Tusken incident; in the Clone Wars, we get a glimpse of his reaction to Anakin's duel with Ventress. However, in ROTS, we don't get any Qui-Gon reaction when Anakin turns, or when he is burned up. Has Anakin's turn lent strength to the DS and thus caused Qui-Gon's influence to "recede" in some way? It could have been developed that Qui-Gon was only truly "accessible" after Mustafar, but at present this is only one possible interpretation, one which contradicts certain EU books.