This may be due to a desynchronisation issue.
If you are on a mid-sourcing configuration, check that the technical workflow 'Mid-sourcing (Delivery counters)' with the internal name 'defaultMidSourcingDlv' is running fine. If its status is NOT 'Started', then start it.
After that, try to recompute the indicators by selecting the relevant delivery in the Adobe Campaign explorer, right-click and choose Actions > Recompute delivery and tracking indicators.
See indicator calculation documentation.
You can also have a look at this technote (PDF format).