I added structured data on a site I manage months ago and although it is getting indexed at the rate of approx 25 items per day the item count doesn't seem to grow. According to the graph it is relatively flat with some days actually seeing a decrease. There are no reported errors that I can see. At the current index rate the item count should have grown to at least 2000+ by now (there are over 100,000 pages in my sitemap) but as there seems to be no historical data other than the graph in my screenshot, I'm having trouble understanding what items have dropped off and why.
Just to clarify, this is not a question about crawl rate. The crawl rate is fine and I can see new pages being added to the index daily. The question is why the item count in the graph is not increasing at the same rate.