It could happen if the bag is too full or if the filters are dirty or blocked in which case change the bag and/or filters and re-test the suction. If your vacuum cleaner has a fixed filter, the filter might need a good clean. If suction has still not improved, there may be a hose blockage. To check, attach the hose to the blower fitting on your vacuum cleaner. This has the effect of reversing the air flow in the hose and should be able to clear the blockage from your hose. If the blower function is not available, one can check for hose blockage by comparing suction with & without hose.