I've played on HackerRank a bit and one thing that held me back from solving some of the challenges was not knowing what I needed to know. If you're going through various courses to come up with the questions, why not link back to those courses so we can learn and then try our hand?
Here's another idea that might help noobs such as myself: have a "tutorial level" or something to that effect, which guides new players through challenges by teaching them the concepts along the way. That'd encourage more people to get involved (you might be scaring some away with how challenging the very first challenges are) and also give people a sense of how to continue with the rest of the levels.