Episode milestones

Can anyone help me with a submission decline please? Unfortunately IMDB aren't overly useful when they put the reason for decline.

I added a trivia to an episode which is the 100th episode of the programme. I added a link in the supporting submission section to a news article confirming it. 

"This is the 100th episode in the Father Brown (2013) series"

IMDB have rejected saying it doesn't meet submission guidelines but it doesn't say which or why.

Any ideas on what the problem is?