I left them a message in the Resolution Center letting them know that the app had ads in it and to please review again.
Man, getting Apple to resolve something that quickly is a Jedi Mind Trick. I’m coming to you the next time I need help with them!
It is not usually that easy. 
I have the same problem. Please, Could you send me the exactly message you sent to the resolution Center? Thanks!
I don’t have that info anymore but I am sure it was very straight forward and similar to what I said above. Good luck.
Mike, Thanks you for your quick answer.
I send : "Yes, the app use some ads but now them are released. I am guessing that for some reason you never saw the ads the first time you open it. Please, could you review it again?
Thanks you very Much!"
I hope it will be ok.
Regards
Just had the same. Perhaps they have a Christmas backlog and some of the (newly hired?) reviewers are inexperienced. As per their request on the rejection notice, I listed steps to show the review team on how to show an ad in the app.
I also had a rejection for an inappropriate keyword. Normally they just edit it out. Maybe they are rejecting rather too hastily in order to meet review time targets?
I have the same problem. Please, Could you send me the exactly message you sent to the resolution Center? Thanks!
I don’t have that info anymore but I am sure it was very straight forward and similar to what I said above. Good luck.
Mike, Thanks you for your quick answer.
I send : "Yes, the app use some ads but now them are released. I am guessing that for some reason you never saw the ads the first time you open it. Please, could you review it again?
Thanks you very Much!"
I hope it will be ok.
Regards
Just had the same. Perhaps they have a Christmas backlog and some of the (newly hired?) reviewers are inexperienced. As per their request on the rejection notice, I listed steps to show the review team on how to show an ad in the app.
I also had a rejection for an inappropriate keyword. Normally they just edit it out. Maybe they are rejecting rather too hastily in order to meet review time targets?