Endnote for ipad downloaded pdfs become "not yet downloaded" after restarting application

As the title says, I have endnote on windows (x6) and ipad application

Before, when I download a pdf by pressing the not yet downloaded button, it downloads the pdf file, and stores it.

However, recently I realized that after I shut down the app and restart app (even after pressing sync before I close), the pdf becomes unavailable.

The weird thing is, usually when we download pdf, the icon for the reference becomes the screenshot of the first pdf page… right? However, there still is the first screenshot of pdf file on the icon, yet the status is “not yet downloaded” I have to download it again.

I have plenty of storage on my ipad, 20 gb left or so, and the endnote app only took 300mb now. but still there is this problem…

any ideas? thanks!