Sometimes a certain region of the map is so much crowded with stars, nebulae, labels, and more, that there simply is no space left for yet another label. In this case the penalty value reaches a very high number. If it exceeds a given threshold, PP3 suppresses that label. You can still override this, see Reposition labels.
You can set this threshold with
penalties threshold 3000
Again, 1000 is the default value.