Do you mean that it never reaches your desired temperature in the house?
If this is the case, and this is a new problem, has anything changed physically with the house (eg, leaky windows, doors, house addition)?
If not, then it could be as simple as a low refrigerant charge on the system, dirty air filter, dirty outdoor unit, or blockage at the outdoor unit. Is there snow in your area? Is snow or ice building up at the outdoor unit?
If it is not one of these things, it could be a bigger problem. Hope this helps. Let us know what happens!