The singer, who won the show last year, signed a joint deal with Syco and Sony back in August 2011 and reached number 54 on the UK album chart with his debut record Believe.
However, McDowall, who met Simon Cowell just once following his victory, was recently informed via his management that Sony would not be renewing his one-album deal.
"I don't think I got as much support as I was hoping to get. When you're on a show like Britain's Got Talent, it's one of the biggest shows in the country at the time. But it got to the stage where I started thinking, 'What went wrong?', especially when you see all these other stars coming out of reality shows," he told the Daily Mail.
"[Believe] came out around the same time as The X Factor was reaching its climax and you had big name stars like Michael Buble bringing out their Christmas albums. I didn't really ask for more support from Sony and Simon Cowell, and I kind of feel like I should have said something.
"I was just going along with it, because I didn't really know what I was doing. I did what I was told. I feel I could have pushed them a bit more, but I think they could have worked a bit harder with the promotion."
McDowall used some of his £100,000 winnings to buy his £52,000 three-bedroom house outright.
He added: "My advice to anyone on the show is try and enjoy it, because I was a nervous wreck. Take it in your stride, because at the end of the day the music business is ruthless. If it's not what you expect, you should be grateful for whatever you do get.
"Things were much better than they were if you'd asked me a year before Britain's Got Talent. You've got to be grateful for what you've got because sometimes life doesn't always work out the way you expect."
A Sony spokesman confirmed the news, adding that the label wishes him well in his future career.
Cowell revealed in January that he was "disappointed" with McDowall, claiming that he wasn't a "genuine star".
Watch Simon Cowell discuss the new series of Britain's Got Talent below: