I have noticed that not all the vents in the room have air coming out of them. Some rooms have air coming out good and some other rooms either have very little or no air coming out at all? Can anyone help?
I'm assuming this is a central air system with ducts all over the house? Should probably go in HVAC.
Anyway, if that's the case, sounds like there may be some dampers that are shut, or heavy dust cloggs in some vents. I would first look for damper handles in the basement and see if some are shut. If they are very old, the handles may actually not even correspond with the position of the damper, so it gets very tricky.
I'm not an HVAC pro, but from pesonal experience there could be several things going on:
1) Closed dampers, as Red Squirel suggested,
2) Dirty filters,
3) Duct runs are too long or HVAC unit is under sized,
4) Crushed or torn air ducts in basement or attic, or wherever they are routed,
5) Lack of return ducts in closed rooms, which hinders air flow.
If you have one central return (possibly in the main living area, and your doors to the other roomns are tight to the floor or carpet, there is no way for the return air to get to the main return vent when the doors are closed. This pressurizes the room and won't let air in from the supply duct. A loop over return (loops over the room door in the attic) provides a path to the main return. This is often easier than adding "home run" return ducts back to the main retun duct.