When you add new Volunteer, Placement and Hours records, keep in mind that certain dates you enter will restrict other dates. For example, if a volunteer didn't start until today, the Reporter won't let you enter hours for that volunteer for last month. The dates hierarchy requires that Stations must start before their Jobs, Jobs and Volunteers must start before their Placements, and Placements must start before any Hours are served at that placement. In other words, a volunteer cannot report hours before that volunteer started or started the job.
A good reminder of the dates' order is to look at the order of the buttons on the main menu: