The short answer as to why Melissa Ponzio appears to be M.I.A. on Chicago Fire is that her biggest storyline is over. When she was introduced on the show back in Season 2, she was Boden's love interest. After some will-they-won't-they shenanigans, the duo finally got together.

Now, they're happily married with a family of their own.

It's a good thing there's no drama between the characters to bring their storylines back to the forefront of the show. Of course, this also means that there's little need for Melissa to make many appearances as Donna.

Boden's wife isn't part of the fire department and she has never had a major storyline of her own. For now, it makes sense for her to remain in the background.