An idea may be to go to the syslog and look for events at the time/s your scheduled speed-test/s did not occur. This may give you some clues perhaps.I've noticed recently that sometimes the speed tests skips in the log. It will just have a null value or something like ---. Does anybody else?
I don't remember it doing that in the past. I thought it might be the server so I went back to random. Still skips a set every now and again.