I have one of these decks that is experiencing a very similar issue. I found on mine that there is a plate to is supposed to slide and activate the auto stop. It is located behind the front faceplate and on the front of the chassis where the function buttons go through the front chassis. The plate on mine got slightly bent where I had to remove it and flatten it out the best I could. While I was in there, I applied some grease to the plate to allow it to slide better. The function is still not 100%, but much improved from where it was. Haven't had the deck back on the bench since as I was a little bit frustrated with it at the time and needed a break from it.
I will say this, the 5020 is one of the best sounding decks I have ever heard. I made a recording from a CD and it was very difficult to tell the difference.
I will see if I can find a picture on the net of the plate I'm referring to.