It is common for the original of an image to come back «Likely AI-generated» while a copy received through a messenger comes back «Inconclusive». That is not a malfunction: evidence was genuinely lost along the way.
Most messengers and social networks recompress and downscale photos to save space and strip metadata for privacy. The C2PA signature, the generating tool's record and camera information all disappear with it.
On a copy without provenance evidence, the detectors judge from pixels alone. Recompression also blurs the fine statistics they read, so more results sit near the threshold and come back inconclusive. For some generators the gap is especially large.
Sending as an original or uncompressed file, attaching it as a document, a cloud sharing link or an email attachment are all more likely to keep the original. Ask whoever made or first received a file that needs checking to send it again this way.
Accept inconclusive as the conclusion and confirm the source another way. A reverse image search for where it first appeared, or asking the uploader for the original, is often faster than waiting for a score.
This is general guidance, not legal advice. A scan result is a probabilistic assessment, not proof.