Weak suction is sometime due to a faulty brush head. Clogged debris prevents it from revolving properly. You may have to reach into the brush head to remove any debris. If you have checked the brush head and everything seems clear the issue may be the motor. If this is the case consider a motor replacement. For instructions on a motor replacement, see the trouble shooting page Shark Navigator Lightweight Upright Troubleshooting
Many of the parts for the Shark Navigator Lightweight Upright vacuum can be found on SharkNinja's website https://www.sharkclean.com under " Parts and Accessories". They sell a wide range of parts from filters to brushes to hoses. For installation of any of these parts check out the trouble shooting page Shark Navigator Lightweight Upright Troubleshooting