To ensure that a buyer campaign is available in the lead flow, you need to publish it in PX through the Source Management feature.
Each lead in PX is related to a source, specifying the originating publisher and vertical. Through Source Management, you can enable campaigns for receiving leads as well as block or prioritize particular lead sources. There are a couple of different approaches for configuring lead traffic depending on the needs of a particular campaign.
For example, the buyer campaign should receive all leads except a certain publisher. You will then need to enable the campaign for the leads traffic and block corresponding lead sources.
Another common situation is when a buyer campaign should receive leads only from particular publishers in selected verticals. In this case, you can publish campaigns with a cherry-pick status, meaning that the buyer campaign will only receive leads from sources that you prioritize.