If your segments are not populating for a long time then please check SMA to see if al the Pega Marketing agents are in running state. You can also launch the System Health landing page from the Pega Marketing Portal under Configuration > Settings menu to see if its reports any errors.
Even we are facing the same issue. When trying to run a campaign using seed list, we are getting this error in the campaign test page itself. Can you help us to resolve this issue without using a segment.
"Failure while creating Data Flow for Strategy Execution"