Did you noticed what differ in this site ? its more related to entertaining.
Most of StackExchange sites are related to education and studies , solving work/school problems. so the motivation will differs from here and there. In this site you have to create something creative and new , there are some 'Point Lines'*(1) to follow them up. Whereas on other sites, if you faced an error on an application or process, copy the error and post it as a question, and the user try his best to find a solution of his question to continue its job,
However here it is totally different, to solve a problem you have to be knowlodge to lot of aspects of fields. I mean a puzzle might contain some technology background, with some knowlodge of Engilsh and math, and try to link points all together.
What I am saying and I suggest, is to form/create a new way to encourage for new comers to interact, and get entertained.
If you go back 5 days and see the questions, youll notice most activity is given to high reputation users, though the new comers having either duplicate or low votes.
What's wrong then with new comers? In my opinion, they are not doing anything wrong, they are just trying to interact, with a simple puzzle they memorised or learned from there parents. They should be encourage and teach them to create new puzzle , we need to motivate them. How to do that? Well I dunno that depends on many things. Beside to down vote them, try to make his puzzle more intersting, if it was really bad or not clear you can send him to meta to an answer how to make his puzzle more clear. Explaining some ideas and ways , give them good examples and bad examples, encourage them in a positive way. Actually new comers will make the site more active.
Point lines: A good puzzle should contain good/funny plot, creative and smart problem , a design.