If your HVAC (heating ventilation and air conditioning) system was sized for the main house, it may not have the capacity to cool your new bonus room. Even if your contractor added air and return vents to the bonus room, there may not be enough force to keep each area cool. This is usually true when the new ductwork was spliced to an existing line.
Before investing in a bigger HVAC unit, make sure that enough insulation was used in your bonus room renovation. If your bonus room is over your garage, check the insulation between the garage ceiling and bonus room floor. A thicker layer of insulation may be enough to keep your existing AC from working too hard as it fights against the heat rising from the garage below.