[Ace]

The reasons are usually pretty obvious. Mobs with dots already applied, current mob dots are near expiration, or you have Garuda summoned and contagion is ready and it's saving Bane for a TriD -> Bane inside of trance.
Some people get too used to using Bane at low levels and don't realize that using Bane vs Painflare at 60 is not always as straightforward, and, more often than not, Bane can be a bad choice unless the target is properly prepped.

So, report back the specific conditions where it is not used and I can tell you more.
If you can only say "it doesn't work", then you'll have to wait until I can analyze it personally.