In my case it was when every thing appeared to be okay on impressions but clicks and rankings were constantly going down. this is when i stopped wondering about updates and begins looking at the pages themselves.
Frankly it does not happens over night to me. What i realized was that pages started receiving impressions but not clicks on key words that were giving me decent rankings earlier that became the something’s not right realisation for me.
it happened when i realized i was receiving impressions but not clicks at all. initially i thought i had to simply create more content.but after checking search console i disscovered many pages that were ranking for keyword searches that did not make any sense.once i realized how important it is to consider intent, content quality nd internal linking rather than keyword searches alone everything fell into place.
Yes, rankings can remain stagnant even though the quality of traffic is deteriorating, particularly in cases when there are changes to SERPs and/or intent. I would look at all three metrics together rather than use ranking as the KPI,