I am not an expert, but have been doing some serious research on the subject lately.
From my understanding, you are much more likely to have a successful spawn if there is at least one other not-too-aggressive fish with the parents (I suspect a pleco probably wouldn't count), as they will guard the nest and eggs, especially new spawners, who can be prone to eating their own eggs or destorying the nest by the parents squabbling.
Do you have info on your GH by any chance?
I do hope others who know more can chime in on this!