Challenge that are super specific limits creativity and so fewer people will be interested. Also your deadline is pretty short, you've created the challenge approximately 4 days ago(most likely August 7,2020) and the deadline is August 15, very short deadline.

Something like WW2 Bomber Challenge would probably attract more attention that Fallout Challenge. WIth WW2 Bomber, you are not limited in creativity!

That happens to every single challenge I try doing!

I think most of the player base is not interested on Fallout themed challenge

From the last month I only see one challenge, that seems to be too niche and vague at the same time. The description never defines what sort of posts are expected in a “fallout challenge”. Now, you did state that it must pertain to the storyline, but it still seems kind of vague to me.
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I mentioned that the challenge was niche, which can be good or bad for challenge performance. In this case it is bad. A challenge that is hyper-specific forces designers to work within a very narrow range of designs. Instead of a fallout challenge, perhaps reframe the challenge in more general terms. Perhaps pose a challenge with a post-nuclear-apocalyptic spin - that would be a very similar challenge with more wiggle room for participants.
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Hope this helps.

The concept must be decent and have a wide variety of choices for example "plane with 9 wings lots of detail has exactly 67 parts less than 1 meter wide challenge" gets no attention while "WWII challenge" gets lots of attention