Discovering the copy predates or postdates what you need.
TOO RECENT
The backup already contains the problem. Common with compromises, which are frequently present for weeks before being noticed.
Check older restore points. Look for the injected content in each to find one that is clean.
If your retention is short, this is where it fails.
TOO OLD
You lose everything since. Sometimes that is acceptable and sometimes it is not.
Before restoring, export what you can from the current state so you can reconcile.
FINDING THE RIGHT ONE
If you have several restore points, check each for the problem rather than assuming.
For a compromise, look for the specific injected file or content in each backup.
WHEN THERE IS NO GOOD DATE
Restore the cleanest available, then manually reapply what is missing.
For a compromise, an alternative is cleaning the current site rather than restoring: replace core and extensions with fresh copies and remove what does not belong.
THE LESSON
Retention long enough to cover a slowly discovered problem. Monthly copies kept for a year cost almost nothing and cover exactly this.
