William:
There are a few options to try:
1a. Start with the default settings and change them one by one to your window's settings to see if you can find the parameter that is causing the problem, it could have to do with mullion location vs glass location/thickness.
1b. Start with the default settings and change 1/2 of them to your window's settings to see if the problem appears, sort of a binary approach to finding the problematic parameters.
2. Add the missing mullions with small column and beam.
3. Convert a copy of the window to a Morph to extract the mullions to add to the original window.
HTH,
David