There's no way for the mods to miss those reports, if they miss it they should be fired because they're not professional. Most of the case when you report the plagiarized post, the mods will delete it, but they will not ban the account especially if the account is high rank.
I can understand about the second chance, but I still remember there was an user plagiarized 7 posts and the reporter post in a different time frame. That's like the mods give the user for third chance, obviously not fair with user which get banned easily by plagiarized one short post.
Handling a report is not an easy task for the mod especially if its related to plagiarism cause they need to be sure that its actually plagiarized content with clear evidence it is going to be easy for them however due to the load they might miss it, imagine they need to review thousands of reports every day for various reasons.
AFAIK, once the mod confirms the content is completely plagiarized with intention not by mistake the post will be deleted along with permanent ban, no warning as far as I remember.