A weak suction motor, full vacuum bag, clog in the suction air path, dirty vacuum exhaust filter or leaky vacuum hose can prevent the vacuum cleaner from suctioning well.
Check the collection bag because debris won't get sucked up through the vacuum and into the collection bag efficiently when the bag is full. Empty the bag if it's full. On a vacuum model that uses a single-use collection bag, you'll need to replace the collection bag when it's full.
If your vacuum uses an exhaust filter, a clogged exhaust filter will prevent suction air from flowing through the vacuum efficiently. Check the exhaust filter and replace it if it's dirty.
Check the suction air path for a clog. Debris in the nozzle, hose or an internal pipe in the vacuum can inhibit suction air flow so remove any debris that you find blocking the suction air path.
For a canister vacuum, holes in the vacuum hose can diminish suction air flow through the nozzle because air gets sucked through hose holes instead of the nozzle. Replace the vacuum hose if it has holes.