I would guess that the a/c unit is not keeping up with the heat gain. You will probably want to put money into insulation as it will benefit you greatly in the heating season as well as cooling. It is not uncommon to have 30+ inches of fiberglass in the attic. Six inch insulation is generally what you would use in exterior walls. Adding insulation and tightening your house up will probably cure your cooling issues. 1.5 tons should be adequate to cool that size of a house. You may also have a refrigerant charge that is adequate which will need to be checked by a competent tech..