Yes, this has happened to me when I was EXP and as PLT. I did call AA again to check on my upgrade request status so both situations it was caught before the day of the flight.
The impression I got from my experience is that getting on a standby list for the upgrade is not simply clicking one icon on the screen for an agent. It sounded like requires multiple screen procedure where require input of proper information at each screen. My impression is that if some information was missing or incorrect, there is no error message to alarm the agent.
One situation, as EXP I requested SWU on a flight to LHR. If I remember correctly I called back few days later to make sure everything was fine, and the agent told me I am on a standby list and all was fine.
Then later when I was passing through DFW, I asked an AAngel at AC regarding my upgrade request. The AAngel looked up my reservation on the screen and at first the AAngel told me I am still on request and everything was fine. However, the AAngel scrolled down the screen and looked my reservation detail further and at the end had some message. There was something wrong with my upgrade request. The AAngel actually showed me the screen, and the AAngel had to go all the way down my reservation at the end to find the note on my reservation.
What turned out was that my SWU upgrade request cleared. But the agent who processed the request made a mistake on putting my credit card information for taxes and fees purpose. Good thing was that my upgrade was still good and all had to be done was to put my credit card information correctly. I have no idea how long the system would have kept my upgrade. If I did not follow up, was upgrade still good the day of the flight? I do not know.
Since that experience I always make sure to call back occasionally and ask an agent if my upgrade request is all good or not. My impression is that good agents will look at the entire reservation detail, which can be multiple pages on the screen. Some agents will just look at the first screen and simply say all is good.