What is wrong with my A/C?

Sometimes when we run our A/C it will run for 12 hours and then it will quit blowing through the vent (but it's still running). The air feels like it's just hovering in the vent. Other times we can get it to run for a week before it returns to just hovering in the vent. The unit is bran new and we recently had it filled with free-on. I don't know anything about HVAC, maybe someone can give me an idea as to what could be going on?