car won't start battery looses voltage

my husband bought a 2006 buick terraza cxl from a dealership whit came with dvd system and remote start and the first day we got it drove it home and went to pick up son a bus stop and car stalled battery voltage went down called dealership they came gave us a jump then when it finally started we took it to the dealership to have it checked out they had to replace the fuel sensor in the gas tank when we got it back we went back home the next day we try to use the automatic start button and the car wouldn't start so we got to get that checked out and then we drove it
down the street to sisters house shut off vehicle and later on when we were going to leave the car won't start its likes the batterys dead as it sits there the battery voltage kept going down what could be wrong