Salesforce percentage related metrics will occasionally display 0 (zero) instead of the intended percent. To fix this, we need to change how the formula is displayed in your report. What we're looking for is this screen:
The trick here is to make sure that this is set to At all summary levels.
To get to this menu, follow these steps:
After opening your report in Salesforce, when in the report customize page, you'll see the formula for that report in the left pane for that report.
Hover over the formula and the pencil will appear. Clicking the pencil displays the initial modal pictured above.