AC is on; however, cooling living room, kitchen, bonus room, all three bedrooms are still unbearably hot. Single level home 1500 sq ft. Any help as to why and what may remedy this would be much appreciated.

The A/C could be to small for the area (you did not state size or model number)

The filter could be dirty

The unit could need service to bring it to its original potential.

There are many possibilitys and with the info provided it is all guess work.