Google Ad Manager's default report for line items shows performance on a day-by-day basis. This can be accessed by clicking the RUN REPORT button in the top right when viewing an individual line item:
Recently a colleague of mine ran one of these reports on a line item, only to find that three days were entirely missing from the middle of the report. The days weren't even listed - the report jumped straight from 6 September to 10 September. Like this:
No changes had been made to the line item around this time - the line item 'change history' showed that it hadn't been paused. So we weren't sure if this was an error with Google Ad Manager.
To investigate, we created our own 'query' with the 'Date' dimension selected and filtering by this line item. The result was the same: three days entirely missing from the middle of the report.
We suspected that Google Ad Manager did not include a line in the report for days with no impressions served. A quick question on the Google Ad Manager Help forum confirmed that this was indeed the case.
Of course, that left us with the separate question of why no impressions had been served for this line item over a three-day period. No doubt a competing line item had taken the impressions, and there did seem to be evidence of a drop-off on the Friday, immediately before the three missing days.
But if you have days missing from a Google Ad Manager report, whether generated via RUN REPORT or the query tool, then don't be concerned that there is an issue with the reporting. There may, however, be an issue with your campaign delivery!