At a certain point in her story, Gwen will decide she needs some space and take some time to herself. At this point, she'll even go as far as to leave your ship to go off on her own little adventure. Finding her again can be tricky, but you should be able to do so if you've followed her story.
The trouble is, if you visit some other islands before going to pick her up, you might notice that she's appearing at some of the Raccoon Inc. stores and not really acknowledging Stella. This is a small bug that can happen where she's still trying to act on her mood behaviors.
To get her back on track again, please go to Villa Maggiore and speak with her there. She should sort herself back out after doing so.