Like I said most of my knowledge on the subject is based on books. That said, any torque converter basically has 3 failure points:
1. The stator or impeller blades or turbine blades self destruct which is a catastrophic failure and your car will go nowhere.
2. The clutch fails to lockup, in this case the car will drive but your MPGs will decrease noticeable as well as the cruising behavior.
3. Or the fluid is out of spec either in volume, temperature, or viscosity.

#1 most certainly require a replacement and since you're vehicle is still mobile this can be ruled out.
#2 could be an electronic issue if the lockup is electronically controlled, which on the 4.6is it is. Replacing the electronics in this system is a hell of a lot cheaper and less labor intensive than replacing the converter so it is recommended that the time be taken to properly diagnose the root cause of the issue.
#3 is self explanatory and as long as the fluid change was done properly with the trans being warmed up, shifted, driven, and level checked/topped off then this has been ruled out as well.

Check out this thread:

https://xoutpost.com/bmw-sav-forums/...stuck-off.html