Curious if you are using Artificial Intelligence on your Trigger qualifiers? You may be getting a triggered event but running it through AI before the alert is sent. And if this is the case then if AI doesn't meet the conditions required to send the PushOver alert you'll never see it. The best way to check this is look at your alerts. If "nothing found" then no PushOver alert if this is how you have it configured. See below....top one, no PushOver alert....bottom one, yes.
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No, AI is disabled on that (and most of my) camera(s). I think Jay is right about the database. It's been a long time (and lots of updates) since I've last regenerated it. That may also explain why the problem got progressively worse over a two week period of time. It was very inconsistent.