There are several reason for your vacuum's inability to pick up dirt. First check to see if there is a clog in your hose. You can try to place a quarter through the hose to see if there is a clog. If there is a clog you can use the end of a broom stick to gently push out the clog. Be very careful with this as you do not want to tear your hose. If the problem doesn't seem to be a clog you may need to take a look at your belt and brush roller.
Most brush rollers can be fixed with a good cleaning. If there is too much debris cluttered around the roller and bearings, it will be unable to turn properly. Since the brush roller is connected to the belt, a dirty roller can overstrain the belt causing it to break or burning out the motor. A vacuum belt assist in turning the roller using tensions. Over time, a vacuum belt will stretch out and lose tension. A vacuum belt should be change every 6-12 months.
The final issue may be a dirty filter. While some vacuums use one filter, more vacuums are starting to use a pre-filter, dust cup filter, and post motor filter especially bagless models. Filters should be changed every 3-6 months. When a vacuum tries to blow air through a dirty filter, only a small amount of air passes through resulting in poor suction.