Did you enter the notes the same way in all parts?  The easiest is to enter them as two half notes and then apply the plugin: TG Tools>Easy Tremolos...

Apparently, TG Tools will not allow you to notate tremolo whole notes in 4/4. It has always been this way, and apparently is an old-fashioned notation convention to notate whole note tremolos as two half-note tremolos instead. I wrote him and asked about it and he confirmed that it was in the design and not likely to change.

Yes, the entering of the notes was done that way, however in a few of the parts the whole note tremolos came out this way.  It is the strangest thing.  I can't edit the stems out either.  I've tried using special tools, hiding the stems, etc.  Nothing works

I ended up writing trills because they happened to work out that way for me, but if the distance is larger than a whole-tone then that won't do in the future.

In a case like that, I would use two sets of (beamed) half notes under a single slur, rather than struggle with eliminated stubborn unwanted stems.