It sounds like you are doing everything right, i.e. cruising at 8kts to the patrol zone.
But in that case your fuel should not be an issue so i think there are two areas that should be noted.
Firstly, is the weather rough? If it is, speeds are reduced or fuel copnsumption is greatly increased to maintain a speed. In rough seas, optimum speed is probably more like 6 knts
Secondly it's possible that once contact is made you are using flank speed for too long. For me flank speed is only ever used for emergencies as it will consume your fuel like i consume beer.
If it's neither of these points, you'd better wait for someone else to come along
Good luck