If your car has been sitting for a long time like years. Check the pulley grooves for rust. When rusty they seem to wear belts very fast. Some brands of belts also may just be too prone to keep stretching. Most good brands just stretch a little and stop. If pulley grooves are basically clean adjust the belt once or twice more. If it continues and your adjustment is locked down try another brand belt. Though this was brought up before so I might look in the archives under belts.