Unfortunately, it is very much par for the course to move down the list, no matter what level you are. However, when people pay for upgrades, if they are already on the list ahead of you, your position on the list would move up. If they are behind you, your position would not change.

You are seeing your position going the wrong way due to new passengers being added to the flight with higher status and/or same status as you but higher fare bucket.

The upgrade list is dynamic, and depending on the route, it may change a lot. I know it's not helping, but there is not much else you can do about it at this point...checking the list is only going to stress yourself out.