I think this is more of a Grow Diaries back-end related issue vs Exotic Seeds themselves.
Exotic does not control how the contests are programed, they only upload a image and sponsor a prize as well pay a big fee to GD to run it.
I think GD put their own rules into play like DOTM where it can only be entered once and after that its done. Kinda dumb when its a brand specific contest, but I'd reach out to GD on the contact us form and go from there.
growdiaries.com/contact