Well if you bought it new then exercise your warranty rights.
If you bought it second hand then loss of suction may be caused by.
Dirty internal filter
Improperly installed bag (if it has a bag)
Blockage in the hose or cleaning head
Split in the hose where air can enter
Blocked dust bag
Gaps in the case where air can enter.
Take the hose off and cover the air inlet with your hand - you should feel the suction and hear the machine rise in pitch as the motor runs faster.
Now connect the hose and cover the inlet hole - the suction and change of sound should be similar.
The results of this testing will tell you if the problem is inside the machine or in the external hose.