Question

I have a 5 ton Trane XL16i heat pump (add on) that was installed 4 years ago. I bought a spec house that had a 5 ton 10 SEER Lenox air conditioner. The air conditioner did a fine job cooling the house, but gas prices took a jump and I was very happy with the Trane HP I had at my previous home. Unfortunately this new heat pump can only keep the house comfortable down to a 40-45F temperature and is inadequate at cooling. Today the temperature barely reached 90F and the heat pump could not keep the house cooler than 81F (on the first floor -the second floor is a couple of degrees warmer). The house is well insulated with 2X6 walls and high end insulated windows. Is it possible the heat pump is not switching to the second stage?? Is there anyway I can tell? A technician checked the heat pump out about a month ago on a 75F day and said the unit was properly charged and temperatures checked out. This house is 3750 square feet (two story) with a very square floor plan.