Have you noticed what makes this site different? Its purpose is to be entertaining.
Most Stack Exchange sites are for education and studies, or solving work/school problems. Participation differs between here and there. On this site you have to make something creative and new. Additionally there are some 'Point Lines'*(1) which should be followed. Whereas on other sites, if you run into an error on an application or process, you can copy the error and post it as a question. Then other users try their best to find a solution to the question/problem. This allows the question-asker to continue their job.
However here it is totally different. To solve a puzzle you may need knowledge from many fields. A puzzle might require some technology background, some knowledge of English and math, and require linking all the points together.
I suggest a way to encourage newcomers to interact and get entertained.
Looking at the last 5 days of questions, you'll notice most activity is from high reputation users. The newcomers have either low votes or their questions are closed as duplicates.
What's wrong then, with these newcomers? In my opinion, they are not doing anything wrong. They are just trying to interact with the community, using a simple puzzle they memorised or learned from their parents. Newcomers should be encouraged and taught to create new puzzles. We need to motivate them.
How to do that? Well I don't know, that depends on many things. Beside downvoting a newcomer, try to make their puzzle more interesting. If it is really bad or truly unclear, send them to meta to ask how to make their puzzle more clear. Explain ideas and ways to improve their puzzle. Give them good examples and bad examples, encourage them in a positive way. Then newcomers will make this site more active.
Point lines: A good puzzle should contain good/funny plot, a creative and smart problem, and be designed well.