I think you have your answer:

Well, I got 50 applicants. Out of 50 only 3 put 'sunshine' and all of them turned to be not as qualified as I wanted them to be.

While this "trick" seems thoughtful in terms of being designed to focus on detail-oriented candidates, it's also way outside the norms of typical hiring processes, and as such you may very well be losing a lot of very well qualified candidates. And, based on your results, it's not exactly filtering the list down to people you actually care to interview.

Ask yourself this:

Would you rather lose a few otherwise perfect candidates because they forgot "sunshine," or would you rather not waste time filtering candidates yourself?

Review your hiring process and make sure your requirements and your benefits are in line with the industry in your region, and - assuming you don't find any smoking gun explaining the lack of applicants, ditch the sunshine thing.

I think you have your answer:

Well, I got 50 applicants. Out of 50 only 3 put 'sunshine' and all of them turned to be not as qualified as I wanted them to be.

While this "trick" seems thoughtful in terms of being designed to focus on detail-oriented candidates, it's also way outside the norms of typical hiring processes, and as such you may very well be losing a lot of very well qualified candidates. And, based on your results, it's not exactly filtering the list down to people you actually care to interview.

Ask yourself this:

Would you rather lose a few otherwise perfect candidates because they forgot "sunshine," or would you rather not waste time filtering candidates yourself?

Trick aside, you really need to do the important work: review your hiring process and make sure your requirements and your benefits are in line with the industry in your region, and - assuming you don't find some smoking gun explaining the lack of applicants, ditch the sunshine thing because it's clearly not working.