Hi Mary,
It could be a couple of things. A dirty condenser coil would be the first thing to suspect. Wait till the heat of the day when it seems like it's not cooling well. Then go to the outside unit and feel the liquid line. That is the smaller of the two copper lines going to the unit. If it's hot, then you need to wash the outside coil with a garden hose.
If it's not that, it's probably low on freon, which is not usually a do it yourself fix.
Let me know if that helps.
Jack
HVAC c&ontractor
26 years heating and air conditioning experience, 14 years running my heating and a/c business.