Copyright infringement warning from Youtube

We're producing videos that will be uploaded to Youtube for presentation. We purchased an audio track from Pond5 and added it to one of our videos as a backing track. Our understanding was that the purchase agreement allowed us rights to use the track for purposes as I have just described. However, the first time we uploaded to Youtube, we received a warning message stating that there could be a copyright infringement issue with the music. Youtube even identified the name of the tune. Then before the video was allowed to post we had to click a button accepting responsibility for infringement should it occur. Bottom line; I can't deliver media to my client if there is even a remote risk that they will see the same message when they try to upload to Youtube. I can't explain such a thing to my client without looking like an idiot or a crook. It seems the purchase from Pond5 was a futile expense of time and money. Otherwise did I miss something here? Because if so please advise.

Here is the Pond5 help article on what to do when youtube brings up the infringement warning:
http://help.pond5.com/hc/en-us/articles/200944663-Youtube-Is-Not-Letting-Me-Use-Your-Media-In-My-Video

You just need to explain to your clients that you have licensed the music and youtube requires proof of license. They will surely understand.

It happened to me hundreads of time with tracks purchased in another (well-known) Company. You just need to click on a button under the "copyright infringment warning" message and in no more than 2 days youtube will close any warning sign on your video. Try walk on their shoes: Youtube have been sued by so many artists in the world. Still, how could they know if you bought the track or just steal it, as milions of people do? I know it seems like a waste of time, but we're all artist and that is the only way to protect our job.

Thanks to everyone who responded for your thoughtful answers. Unfortunately, for this client I have to use content that will not require explanations. Thank you again for taking the time to respond. I appreciate your consideration. DP

Sorry to hear about your luck with the client, Dnews used one of my clips (not audio) on youtube, I love seeing my work on youtube channels, as long as it's does not still have the P5 logo on it, lol...

P5 would probably gladly give you some thing to show the client if need be. They are really great when it comes to working with buyers.