Buyer networks and Ad Exchange reports on Bid Landscape
Some buyers don't appear in your Ad Exchange report on Bid Landscapes because they have chosen to not reveal their bid data to publishers.
This can occur for a variety of reasons, though it is most often due to specific contractual agreements that buyer networks have with their customers that prevent them from sharing data with a third-party.
Buyers can choose to not reveal their bid data at any time. However, once a buyer has opted out of disclosing their bid data, this information is excluded for all publishers they transact on.