Which version of Lightroom Mobile are you using? What is your device specification? Did this start after the system/ application update?
Can you log in to the cloud, lightroom.adobe.com, and check whether your assets are synced?If it's synced, try logging out of the application, uninstall it, and reinstall it again. (Don't do it if the images are not synced)
Certain features like Select Subject and Select Sky in Masking are supported only on Android devices with more than 6 GB RAM. Technology Previews (features that may not be completely production-ready yet but available to try out) are allowed on devices having between 4 GB and 6 GB RAM. Still, you could experience an occasional crash under memory pressure. On devices with less than 4 GB RAM, these features are unavailable.