zanussi zwf14791w program problem?

can you help when i put my zanussi zwf14791w on to wash it shows 2hours 30mins,the machine will be finished in less than half this time shown and the washing powder is not all used there is still half the washing powder in the drawer,THIS ONLY HAPPENS ABOUT EVERY SECOND OR THIRD WASH its not every time ,,thanks hope you can help,A.R.