Unity Banner Ads not showing if I specify a placement id

Hi,

I am trying to integrate the banner ads. It is working fine if i use the default banner ad. But once i specify a placement id (which i created through the console), the status is always "Not Available" when i am running within Unity. Has anyone seen this or have some idea on what's wrong? thanks so much
(I am still on Unity 5 btw)

regards,
Ting