Make sure all the tanks are seated properly and all clamps are securely clamped. I found that if it's not all sealed tight it won't suck. Also make sure the red filter is clear debris. You can hear the difference when it's all tightly sealed. Hope this works for you.
Sounds like you might have got the wrong belt, it should be snug but not super tight. Take belt off and see if the motor spins freely, if so I would try another belt.
It could be one or more of many things. If the brushroll is restricted the motor may be overheating. There could be a bad switch, connection, loose or pinched wire somewhere or the motor is not getting proper voltage. The motor brushes could be worn out.
