Update: Apparently it's all working normally, as I got the following through from AdSense technical support...
Please note that reports on impressions and clicks are usually updated every 15-30 minutes, but occasionally there may be a delay of 24 hours or more before your reports are fully updated. We assure you that your clicks and impressions are being tracked even if they don't show up immediately on your reports.
Which, while perfectly true, doesn't really seem to explain the "No data available" messages. I've certainly never seen that before..?