Based on our investigation, we found that some journals or scientific articles with DOIs from Zenodo are not indexed in Google Scholar. This phenomenon can be caused by several technical and procedural factors, including:

Indexing Time
Google Scholar requires a certain amount of time to index new content. Articles that have recently been published, even though they already have a DOI, may not yet appear in search results because the indexing process has not been completed.

Site Configuration (robots.txt)
The robots.txt file on the Zenodo site may restrict access for Google Scholar bots. This could be due to configuration errors or intentional decisions for security reasons, preventing Google Scholar from crawling and indexing those pages.

Metadata Structure and Completeness
Google Scholar relies on complete and well-structured metadata, including title, author names, abstract, and more. If this metadata is unavailable or not formatted correctly, the article is unlikely to be detected by Google Scholar's indexing engine.

DOI Validity and Accessibility
An invalid DOI or one that does not directly link to the article's content can cause Google Scholar to fail in the indexing process. Ensure that the DOI used is functional and directs to a legitimate and open publication page.

Publisher Policies on Indexing
If the publisher or distribution platform does not actively manage the indexing process on Google Scholar, the article is unlikely to be indexed, even if it meets other technical requirements.

Recommendations for Authors and Researchers
Before deciding to publish scientific work in a specific journal or repository, ensure that:
1. The DOI used comes from a credible publisher recognised by Google Scholar.
2. Article metadata is properly structured and complete.
3. The publisher has a track record of indexing in Google Scholar or other scientific platforms.

By considering these factors, you can enhance the visibility and impact of your scientific publications.