A/C won't cool down upstairs of house...?

My family just had a new A/C unit installed about two years ago, and it works great to cool the downstairs of the house in the summertime. The only problem is that the upstairs seems to be immune to any cooling. You can feel a very weakly blown amount of cold air coming from the upstairs vent, but the effects are almost negligible. Is there something I can do or try in order to remedy this situation? Any suggestions would be amazing.