Hi Hamish_TM,
It is hard to say without knowing the exact URL but here are some things to consider:
- Indexing Lag - How long ago did you submit the sitemaps? We usually find there can be at least a few weeks lag between when the sitemaps are submitted and when all the URL's are indexed.
- Internal Linking - What does your sites internal linking structure look like? Good internal linking like having breadcrumbs, in-text links, sidebar links and siloed URL structuring can help the indexation process.
- **Sitemap Errors - **Are there currently any sitemap errors listed in Google Search Console? Either on the dashboard or in the sitemaps section? Any issues here could be adding to your problem.
Hopefully, this is of some help and let me know how you go.
Regards,
Jason.