HI @matrixstar You may need to reset phone/PC or unlink your device. see more information here https://support.microsoft.com/en-gb/help/4531298/troubleshooting-the-your-phone-app
I am not sure which Windows version you are using but if Windows 10 and still problems you may need to check additional settings. https://www.windowscentral.com/how-fix-common-problems-your-phone-app-windows-10
News that Windows 10 PC's will be able to launch Android apps from many Galaxy Smart Phones via the Your Phone app, gradually rolling out. The Apps and mirrored phone screen will launch in separate Windows. However there may be certain issues as some apps may block the ability to cast to other screens, some apps may not respond to a mouse and keyboard, whilst the audio on some apps will be via your Galaxy Device rather than your PC. Therefore a work in progress. An upcoming update whereby users will be able to run multiple Apps side by side will firstly come to the Note 20 series .
Frequently Answered Questions Here: https://answers.microsoft.com/en-us/insider/forum/all/apps-feature-your-phone-app
I am finding the Your Phone App working much better nowadays . Running Multiple apps seems to be working on my S10 (though just checked it a few times) However not all Devices running Android 11 are supported. https://www.xda-developers.com/your-phone-multiple-android-app-support/
For some 2000 Smartphone photos have been supported on Windows 10 https://www.thurrott.com/windows/windows-10/225587/your-phone-to-support-2000-smartphone-photos