An ad doesn't appear every time a user searches, and ads cycle between other devs bidding on the same terms. After a certain amount of ads being displayed and a user showing no interest (or simply a user being shown a certain number of ads for one search), these seem to dry up altogether.
Do you see any ads for the search term from other devs? Are you getting any impressions reported at all in the Search Ads Dashboard?
If others have ads showing, but you don't, maybe your bids are too low. If noone else seems to bid on it, maybe it's not a popular term or searched on at all by users. Or maybe Apple have decided that search term isn't relevant for your app.
How they work out relevancy is a black box, but if your term doesn't appear in your title, keywords, or app description I've found a lower probability of that term generating impressions. For instance, I created a campaign testing potential new keyword terms, and it has been woefully poor at generating impressions, even with high bids. I suspect the phrases were deemed not really relevant to the app in question. My other campaigns more closely aligned to the sort of keywords you'd expect, have been much more successful.
