Engineers and Bunker Construction Question

[XLVIII Pz. Korp]

I'm currently playing the scenario Flanking Kharkov. (PB Demo)  As the Germans I am digging in.  Several improved positions already upgraded to trenches.  I have engineers in position attempting to start digging in further to upgrade those trenches to bunkers but every time I order the engineers to dig in I receive a message that the engineer can't dig in when a fortification exists  in the hex already. I am confused...  the manual states: 

Bunkers are the next step up in Fortification development. Bunkers can be a part of a scenario or may be created by digging-in during a scenario from Trenches by non-Bridge Engineers.

And this is exactly what I am attempting to do, issue the dig-in command to a non-Bridge Engineer in a hex that contains a Trench. And all I get is a message about not being able to dig in where a fortification exists.

Can anyone enlighten me as to what the heck I'm doing wrong?  Has anyone been able to construct a bunker during a scenario?

Thanks!