Manuvers not working?

[LiquidFuelBoosters]

I'm playing on sandbox trying to get a Duna encounter, and managed to get one with a maneuver node. Then I set my SAS to "Maneuver" and waited until the maneuver said - 15s, 14s, 13s, and then burnt once the time disappeared, like you would do with any maneuver node. The maneuver, however, did not work. The est. burn kept changing randomly, the amount of delta v on the bar on the right of the globe kept changing up and down, and the icon on the map kept moving around randomly, therefore my ship followed, ruining my mission. I'm really annoyed about this, and I REALLY NEED to know how to FIX this so this NEVER happens AGAIN.

Thanks, in advance.

[Master Tao]

A few possibilities come to mind:

• Did you have available fuel?
  
• Did you have an active engine?
  
• Was the engine facing the right direction?
  

I ask because so many of us have experienced at least one of these problems. Otherwise, a quicksave right before causing the problem would help.