Brands and agencies are investing more in private marketplaces for better inventory. But in practice, it is hard to run a private marketplace deal smoothly.

A PMP works like a private club where media buyers — with a membership card called a deal ID — can run invite-only auctions that are usually more expensive than open exchanges. In its simplest form, a PMP deal takes priority over open marketplaces, but it falls below guaranteed business (sponsorships and insertion order-based sales) in a publisher’s ad server. (Header bidding and supply-side platform integrations can change such order, but that is a different story.)

For Charlie Fiordalis, chief digital officer of media agency Media Storm, insufficient inventory in PMPs is a big issue. “Sometimes there is not enough inventory for us to bid, especially when we run campaigns in a very short period of time,” said Fiordalis. “In that case, we need to be good friends with the publisher and say ‘Can you please prioritize our deal ID and clear our X impressions in the following three days?’ It does take some manual work.”

“It’s a double whammy of data and can dramatically decrease scale,” she said. “Data is awesome, but too much of a good thing can be a huge challenge with PMPs.”

And it could simply be buyer configuration issues where publishers don’t receive any buyer activity, while on the buy side, the deal bids may not go through because publishers are not sending anything biddable, according to people interviewed for this story.

Sometimes the data does not match up because the publisher’s first-party data — like subscribers and newsletters — is actually second-party data to the buyer, said Erik Requidan, vp of programmatic strategy for marketing consultancy Intermarkets.

For instance, based on its first-party data, publisher X thinks that advertiser Y is able to acquire 10 car shoppers in a PMP deal. But after advertiser Y applies its own first-party data it may only find three eligible car shoppers from publisher X. The situation can be worse if advertiser Y adds other targeting parameters like gender and age.

“If that’s the case, the deal is flawed at the very beginning,” said Requidan.

There’s also a lack of trust between buyers and sellers. Media buyers lack verification methods to hold publishers accountable for their targeting data and the inventory priorities that they claim.

But at the same time, advertisers typically don’t give publishers their campaign parameters, so publishers are in the dark in terms of what they are driving throughout the campaign, said Julia Welch, vp of enterprise engagement for MediaMath.

From an agency’s perspective, Media Storm’s Fiordalis explained that if advertiser X wants to purchase impressions from publisher Y of in-market car shoppers, publisher Y does the data work on its own and advertiser X has to trust that the publisher will push through the correct inventory.

“We don’t like to trust people that they’re doing things right unsupervised,” said Fiordalis. “We like to have full visibility so we can verify that the inventory that is pushed through a deal ID for a PMP is exactly what we purchased. For that reason, we don’t include any data parameters in our PMP deals.”

For better targeting, Jon Mansell, vp of marketplace innovation for Magna Global, thinks that the buy side should push its tech providers to help provide more transparency to publishers who usually don’t know when and why various tactics are bidding.

“Programmatic sellers need such data to sell more intelligently,” he said.