missing availability, response time

Hi I am monitoring wireless access points using NAV 3.5.4. Some of these access points only report "N/A" for Availability and response time. This is a similar issue to bug #345783 submitted by Yours truly, but not identical, because here there is no data at all, while bug #345783 addressed an issue with certain time periods being empty. It may be related though... I have looked in the rrd_file table, and there are entries for the relevant devices, as well as rrd files in the rrd folder. Is this a bug or a seeding error? Thank you! Lars