Most normal carpets need to be vacuumed with the brush roll on in order to effectively remove dirt from the fibers of the carpet. If it has been more than a week since the carpet was vacuumed, or it's in a high traffic area, carpet may need to be passed over 2-3 times. If your carpet is more delicate, you may need to vacuum on the bare floor setting, or vacuum with an attachment wand.
How Often to Vacuum Carpet:
Areas that receive a lot of foot traffic will need to be vacuumed daily or twice weekly, depending on the number of feet walking your carpet. Other carpeted areas may only need to be vacuumed once a week if they receive less traffic.