- Target set too aggressively. Google will show nothing rather than bid where it cannot hit your goal. The most common cause by far.
- Budget exhausted earlier in the day than you noticed.
- Disapprovals — products or ads removed from the auction entirely.
- Billing problem — a declined card stops everything and the email goes to an address nobody reads.
- Audience or location too narrow to serve.
Check in that order. Four of the five are visible in under two minutes.