AI People masks only working on some images

is your GPU perhaps by AMD, if so perhaps see:

https://community.adobe.com/t5/lightroom-classic-discussions/quot-select-person-quot-ai-feature-does-not-work-in-lightroom-with-amd-24-1-1-drivers/td-p/14383035

Also

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut out as that is just so much dead space to us non-Techs and it takes up vast amounts of scroll space making the reply less readable and less likely that others will bother with your post.

Can you share two things?

• A screenshot of your Library Module, in grid mode, with the CR2 image that fails people mask selected. Full screen, desire to see navigator, folders panel,  grid, histogram, and filmstrip. Preferably with filename visible. 
• A screenshot of your Develop with that same CR2 image, with the create mask panel visible, perhaps with an attempt to select people mask