This is the most common issue experience by all new publishers as well as existing one.Only few reasons are behind it. Check out the below mentioned things are done correctly on your end.
Commonly committing mistakes:
- Creating video tags from the Google Ad manager and inspecting the tag without targeting any line item to that ad unit.
- Creating video ad units with one video ad size and targeting line item with different ad size or different ad unit.
- Targeting to the specific Geo and checking with different Geo location.
- Adding Frequency capping in the line item level or ad unit level and checking the VAST response.
- Targeting the video ad units in the line item with key values and forgot to add that key value as custom variable in the VAST tag.
- Most highlighted mistake is missing random number macro in the VAST tag in place of Correlator or Timestamp
- Make sure the line item which you targeted having enough impression goal to deliver video through VAST.
If above is rectified, everything is fixed.
Hope this helps!!!!!. Thank you!!!!