Washing machine drum won't turn when loaded

Washing machine APM8512 Diplomat (front loader) the problem we have is that the drum stops turning when loaded and when full of water . The motor is trying to turn the drum but doesn't seem to have enough power to do it. It can spin OK and will work with no washing just water. We have checked the belt is OK, have changed the motor and checked that nothing is trapped inside the drum. We have now run out of ideas and having spent out quite a bit on the unecessary motor would ask if anyone has any ideas?