Sometimes a plate misses an inspection, even though the system seems to be up and running. This could be a result of a combination of imager logic, which says that an inspection is skipped if it can’t occur within 24 hours of its scheduled time, and an uneven loading of plates to be imaged.
Assume that your plate was set up on a Thursday. It turns out our inspection schedule is such that a plate set up on a Thursday will have many of its inspections scheduled for Thursdays (as the later inspections are done weekly).
Each inspection of a plate takes some time – the first inspection is a little slower than the usual, as the imager has to find the drops the first time it sees a plate. The 5th (7 days = 1 week after setup), 10th (28 days = 4 weeks after setup) and 15th (70 days, or 10 weeks after setup) are very slow, as they include a UV inspection.
Each plate inspection gets a priority: more recently set up plates are imaged before older plates.
So going back to your hypothetical plate that was set up on a Thursday: if, by happenstance lots of plates were set up on a subsequent Thursday the bulk of imaging these would also on Thursdays, and your plate (being older) would have lower priority for imaging, and might skip an inspection.