I live in MD. Bought this house in 2005 brand new - 4200 sf including finished basement. Insulation is good. Have a 3 ton Trane for the main level and basement and a 3 ton Trane for the 2nd floor. I've been told they are 10 Seer. The inside unit for the bottom levels is in the finished basement and the inside unit for the top level is in the attic. I've been told that my duct work is fine.

My problem is that when the temperature gets above 85 outside and the air conditioning is set at 73, the temperature in the house never gets to 73; it rises all day until the sun goes down. In the winter when the temperature gets in the 30's, the emergency heat runs all day.

I've had several people out to look at the problem and most (unless I tell them not to say it) say to me - 'well, it is hot outside'. In my old house built in 1992 (3100 sf including finished basement), I had one 2 1/2 ton unit to take care of the whole house. If I set the temperature at 68, no matter what the temp was outside, it would get to 68 eventually. It would not rise!

Can someone please tell me what the problem could be? Or what I need to fix the problem?

Have you inspected or questioned the insulation?
You might want to consider closing off the vents supplying the basement,and see if that helps the mail level
You may also want to have a tech check the freon level to see if its where it should be,, low freon would effect cooling and the heating in the winter

Heatpumps are not meant to run in below 30 degree weather and a lot of times if it is that cold out the heat strips will activate to assist in heating the house. As far as the cooling do you change your filters regularly? Also you may want to have a full maintenance done on the system. i.e. freon check, clean outdoor coils, go through all components.

Anther possibility, your heat loss and gain are too great for those units. In other words, you are undersized.

The best way to verify is have a load calc done - otherwise known as a Manual J. You can do it yourself by purchasing a copy of HVAC-Calc.