You should be getting some additional information along with that message - a link to the Policies ( http://www.google.com/intl/en/+/policy/content.html ) and a link for reconsideration request. Apply for reconsideration.
The most likely reason according to me is that someone has reported your post. See the answer by a Google employee here : http://productforums.google.com/forum/#!topic/google-plus-discuss/oq4rVu34QJo
we manually review when the Google+ community reports content through our Report abuse feature, and the appeals process varies in time depending on the violation and other factors. If you appeal the violation on 2 posts, only those 2 posts will be re-reviewed by our team.
It might also be an automatic block based on heuristics like posting excessive links.
Also, according to their Terms, holding contests on G+ pages is not allowed.
Can you tell what is the content of that Google+ post which is being blocked?