I have performed many demos for Chalk, as I work for Rockwell. I use my phone, and the desktop app, to show the session. sometimes the desktop app does not allow for me to draw an AR object in the desktop app
This would depend on the device sharing their view. If the local user does not initialize their environment, or are using an unsupported device, then Chalk marks will not work. See this community post for more information.
Are you consistently receiving the UX guidance to initialize your environment? You should see both messages like described in this Help Center topic: https://support.ptc.com/help/vuforia/chalk_desktop_center/#page/Vuforia_Chalk_Desktop%2Fcommon%2Fini...
If you aren't, please let me know as it may be a bug.