1. What caused poor / low suction
Low suction is usually caused by blockage in the pipework. It can also be caused bya leakage, but blockage is most common. To know whether it is a blockage or a leakage, put your hand over the end of the curved wand to block the hole, and listen to the engine noise. Blockage will generate rising noise from low to high and then stable at high noise level. While leakage will have same middle level noise.
2. How to unblok a blockage ?
To unblock a blockage you will need another vacuum cleaner (eg a portable cleaner) to suck back the dirt from ducted vacuum inlet valve.