There are two straightforward explanations that are possible:
The application transaction was not keyworded. We recommend that each page of your transaction be keyworded. If the transaction wasn’t keyworded, and if there was a failure, it is possible that the custom error page did not trigger any Deja Status 97 errors as the custom error page may appear similar to the original page as well as send 200 OK (and that no HTTP 404 was sent.) It is for this reason that we always recommend a keyword on each page. If a keyword cannot be found, a Status 5 keyword error will be declared.
Here is the relevant help section to walk you through keywords using FireFox. (The process is nearly identical with Chrome):
Another reason is that the monitoring interval may have been too long. If you are running the transaction monitor every 15, 30 or 60 minutes, it is possible that the application error occurred in-between monitoring measurements. It is for this reason that we recommend monitoring at more frequent intervals, in order to be able to catch the application while in error. This is especially important for e-commerce applications, where e-commerce revenue may be lost due to a short but undetected outage. We have customers that run every minute for this reason.
It is a very simple procedure to change the monitoring interval. Here is a short video that shows how to change a 60 minute monitor to a 5 minute monitor in UXM.