Because the MyPMS system allows guests to book a 'Room Type' and not assign an actual Room until closer to check in, there can be times when the Tape Chart availability looks off or shows a 0 or -1, etc.See Tape Chart
This is usually it's due to a reservation that has been booked into a Room Type, but no Room Assignment has been made.See Room Assignment
There are several ways to find the bookings with no Room assignment.
Start by going to BOOKINGS|SEARCH BOOKINGS See Search Bookings
Once you have created the list you need:
If there is still a question about availability, then it's also a common situation that a Group Booking has a Hard Allotment that is pulling availability. This will show on a Search using 'Begins on:' and 'Ends on:' as well. But if you wish to only view Groups, go to Bookings | Group Bookings and search on all Groups to see what dates of a Group might be holding back availability for the dates you are analyzing. See Group Bookings: Rooms Allocated Exceed Available Rooms
If neither of these strategies work, we can try to help through http://support.bookingcenter.com/, entering the Room Type and dates in question into the ticket, and we will try and assist.