Demotivated and evasive
The person you’ve enlisted to your shared project has become despondent and is avoiding you. In meetings, she goes through the motions. Enthusiasm has evaporated, and there’s been a “mood” for a few weeks now.
An instinct of British reserve might be to pretend it isn’t happening, to bury oneself in one’s own work, and hope she snaps out of it.
But what if she doesn’t? What if it is putting the project in jeopardy?
Better to address the issue politely but directly. A challenging opener would be: “It feels to me that things have gone flat around here, Beth. Is everything all right?”
Be ready, though. She may be having a personal crisis, or is about to leave for another job. It may also be that something about your approach is causing her problems.
The sooner you know, the sooner something can be done about it.
If she is still evasive (“Really? What makes you say that?”), have some specifics to hand, such as agreed tasks not done.
Emotions might elevate, but that’s okay. Your job is to unblock the lines of communication, which are essential to delegation.