Missing product pages in shopify sitemap file

[user072319]

We have an app which reads the clients shopify store's sitemap.xml file to get all the store links. But for some of the clients we saw that in the product sitemap lot of products are missing. 

<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<!-- This is the parent sitemap linking to additional sitemaps for products, collections and pages as shown below. The sitemap can not be edited manually, but is kept up to date in real time. -->
<sitemap>
<loc>https://domain.com/sitemap_products_1.xml?from=1528132862065&to=4693895020624</loc>
</sitemap>
<sitemap>
<loc>https://domain.com/sitemap_pages_1.xml</loc>
</sitemap>
<sitemap>
<loc>https://domain.com/sitemap_collections_1.xml</loc>
</sitemap>
<sitemap>
<loc>https://domain.com/sitemap_blogs_1.xml</loc>
</sitemap>
</sitemapindex>

I checked the sitemap_products_1.xml and found lot of product pages are missing in that file. Why those are missing and how can we get those missed products? We want all the product links from the sitemap for our app functioning. Does the store owner needs to add those missing products to the sitemap?

Any help would be appreciated.

[jam_chan]

Shopify automatically added generates sitemap for all stores. When the content is changed, Shopify will regenerate a new one. However, the sitemap.xml file cannot be changed

[markusbrandes]

It's funny that there was no more feedback to your question. Now quite some time later I see that products are mission in the sitemap files as well. Is there anything new to say about this issue?