A family of Microsoft relational database management systems designed for ease of use.
I had this very same problem just now. What it turned out to be was the Report Grouping.
While in the Report Design Mode, left-click the ruler bar on the left side of the report and select Sorting and Grouping. If your Detail section falls under a field name's group, it could be causing the issue.
I noticed that I could get the banding under the field name group, but not the Detail group. Luckily in my case the field name that I had it grouped under was arbitrary (I had it grouped by record ID, which is unnecessary since every record is technically grouped by this), so I was able to just delete that grouping and then my Detail section had banded rows again.
Hope this helps.