In my experience, when editing a challenge piece the challenge box isn't available. 
Too bad because that was a great piece.
All i changed was the preview since one of the artists here mentioned that the "preview" text wasn't pixel art because i placed a drop shadow on the text. I was afraid that my entry would not be accepted so i changed it. I think it automatically unchecked the button when i edited it....Sad.
Really sorry this happened to your entry this week.
The weekly challenges are automated and this has never happened before that I'm aware of so you must have not checked the challenge box and only thought you did.
In future entries after the pixel is added chack the page to make sure your entry is added or contact any Mod before the deadline to check for you.
I see on the art page that you edited the preview after the art was added.
Its possible you unchecked the art at that time ???
I was one of the first people who submitted even. I didn't forget to check the box as well...What a waste of time.
 |  |  |  |
waaaa O.O