You often won't see ads not because they are not live, but due to logistics resulting in them not displaying to you in that specific instance. Geo, device, day parting and other targeting settings may result in ads not showing to you while live and showing to other users. As SERP
results are increasingly customized, even at full impression share not all users may see ads. Engines increasingly consider inferred intent, search history and other factors when determining if to show ads besides campaign keywords and bids. Of course, depleted budget or disapproved status are other common reasons for ads not showing up.