The problem is getting a handle on atmospheric density at the orbital altitude. Since they can’t do a controlled de-orbit burn the satellite is at the mercy of whatever traces of atmosphere exist at it’s current altitude. Without a way to precisely measure that at all points in the orbit, there is no way to precisely determine how much it is slowing it down until after the fact.
They can make estimates based on tracking (which is exactly what they are doing), but cannot tell exactly what part of the orbit, or even exactly which orbit it will give up the ghost and re-enter.
Thanks for that explanation, friend.