How to get a/c to 2nd flr?

We have central air/central heat which (ducts) comes from the basement and we are unable to get any relief on the 2nd flr. My husband has spoken to several people who say we made need an a/c booster to get the air forced upstairs. We have no problems with the a/c on the 1st flr, but some of the vents upstairs (2nd flr) either don't feel any air at all or do but it doesn't make a substantial difference; it's very frustrating. At night we turn up the a/c in hopes that it will better cool off the upstairs, but it doesn't. We also use ceiling fans..could it be that we don't have enough vents as well? (our vents are on the floor, one in each room - 3 rooms upstairs and a bathroom)..