When mmhmm Studio cannot recognize an app window that it cannot "see", it may not display your screen share slide.
Minimized app windows
App windows that appear minimized in your dock will not be available to screen share in Studio until they are restored.
Virtual backgrounds in your video conferencing app
Zoom, Google Meet, and several other video conferencing platforms offer the ability to apply virtual backgrounds to your webcam. This will prevent your audience from being able to see your mmhmm slides.
To resolve this issue, check that the virtual backgrounds feature is turned off in your video conferencing app: