I can't seem to understand whether the issue is with the advertisers pulling their ads from specific videos or whether YT is just having an issue with inventory. I believe that if advertisers have decided to pull their ads from your videos, then YT owes it to the content provider to let them know this. I understand their discretion to maybe not explain why, but at least the channel creator should be told that there are no ads to serve.
I think it's poor customer relations to tell the YT account holder that their account is "monetizing", but there are no ads to actually allow it monetize it. Just be open and say we screwed up, don't have enough ads to spread around, and we noticed your account has not been on the list of videos to get this ad. Enough of sending the customer to a stupid blog and have them try to guess why this is happening.