2005 Avalanche will spin over but won't crant

I have a 2005 chevy avalanche that I just bought this past tuesday. When I test drove it it cranked fine. After I signed the paperwork to purchase it and got ready to leave the gas light was on due to low fuel. It acted a little hesitant about starting and I figured it was because of low fuel. Went straight to the gas station and put $30 gas in it. When I got ready to leave the gas station it still acted hesitant about cranking. It would spin over but acted like it didn't want to crank. On my way to another town to pay a bill the check engine light came on. Took it by auto zone and the computer said it was a knock sensor. Got ready to leave and it spun and spun and spun and finally turned over. Went back to the dealer I had just bought it from. They put it on their computer and got the same reading (a knock sensor) they replaced the sensor. Got in the vehicle and drove it home. Went to leave home and same problem vehicle would spin, spin, spin and not want to crank. You could turn the key off and on several times in a row trying to crank it and it would finally crank. Every time you drive it it takes longer and longer for it to finally crank. Any suggestions. I'm so fustrated.