re: Central air not cooling in the daytime

bet it's a freon leak and low on freon

Change the Flux Capasitor.

My unit did this last summer. It turned out that my drain line had become clogged with algae, and water was going into my drip pan. The drip pan had a float switch which killed the compressor when the water level got too high. I emptied the drip pan, put some drano down the drain line, and it worked fine.

could be a capacitor, takes about 4-5 minutes to change, around $30 @ hd

Not a lot of help in this thread so far.

First, do you have ice on the line from the outside unit leading to the upstairs coil? If you do, you likely are low on coolant.

Second, when the fan on the ouside unit is on, is the air blowing up out of the unit hot or is it normal temperature? If it is normal temperature, it could be a blown capacitor.

If it cools at night, and it is neither of those two issues, it might be either your inside filter was old and preventing adequate airflow.

Also, the unit outside needs to allow air to get the blow off the heat. If there is old grass clippings or old leaves or just about anything blocking the airflow around that outside unit, it will not be able to work well.

So as someone said you can always try cleaning that unit of any obstruction.

Those are all of my hints from my limited knowledge of AC units. And you should double check anything I say with someone who knows what they are talking about...