If you are using Lightroom on multiple devices, are you sure the images have finished importing from another device such that the photos and their smart preview copies are actually ready for editing on peer devices?
You may have issues that are stuck during sync. If you sign into your same account at lightroom.adobe.com, do you see an album called Sync Issues with photos inside? If so, 1) be sure have you backups of the originals stored safely outside of Lightroom, and then 2) delete the affected images. Finally, 3) re-import the images.
If incomplete uploads/sync issues aren't the reason why your Lightroom Mobil app can't edit the image, does it help if you make simply add a caption in Info view? I wonder if metadata isn't flowing well, and pushing other kinds of metadata might unblock the image.
Please let me know if anything above, or if any other steps you've tried on your own help.