Why does my google index status (sitemap Indexed) go down?

I add six pages a week average and update my sitemap weekly. And google used to match indexed page to submitted links by sitemap within few days. Now for the last few weeks it sort of started a reverse process. I submit more pages and indexed status drops few pages a week.

All of my contents are original. I don’t have duplicate page other than on which I got rid of. does this happen to someone else and how can I fix the issue.

Any help would be highly appreciated.



thank you so much