SeboU1, the mistake you made with the Riccar Spirit was using it to pick up the Sebo Duo P powder with a relatively full bag! When you're picking up that kind of fine, powdery dirt, you need all the airflow the vacuum has to get it out and into the bag.
Next time, try putting a new bag in the Riccar, and I am sure it will pick up the Sebo Duo P powder MUCH BETTER. Also, one little tip you can do to restore airflow and suction as the bag fills is to take the bag door off, give the bag a good vigorous shake while still mounted to the bag fill tube, then close the door and check the suction at the hose. You would be AMAZED how much suction is restored simply by shaking the fine dust off the bag walls.