Resolve Site's Google Indexing Issues

My website is experiencing significant indexing problems with Google, where certain pages are not being indexed at all. I've checked Google Search Console for manual actions or penalties, but I'm not completely sure about the status. However, I do have a sitemap submitted to Google Search Console.

Key Requirements:
- Diagnose why certain pages are not being indexed
- Check for any manual actions or penalties
- Optimize indexing process

Ideal Skills and Experience:
- Expertise in SEO and Google Search Console
- Experience in troubleshooting indexing issues
- Knowledgeable in website optimization strategies