There are two common causes of suction loss, either the filters or the vac hose is blocked. Here is how to fix this problem:
Unplug the machine.
Remove the filters and clean them properly, making sure you are following the manual instructions. Some filters can be washed while others may only need brushing.
Detach the hose and check if there is any blockage. Also, check the base of the vac for blockages and make sure that the belts are not damaged.
Bagless models require regular emptying of the dust container to keep maximum suction whereas, for bagged devices, you just need to check if the bag is full.