Book doesnt show sometimes with load image showing indefinetly

[mahag]

Hello
I have been using readium-js-viewer for the last few years inside my webpapp to allow users to read epub files on browser.
Many users face the issue that when an book opens the load image displays indefinitely and book content doesnt show as in img attached

Sometimes the same book opens and sometimes it doesnt.

-The file does exist in correct location on server and has .epub extension,no space in epub_content folder and subfolders.
-Internet connection is available on user 's machine.

• Table of contents does show even when load img is displaying and when we try to click on a submenu from table of content nothing shows.
  -Browser used is Google chrome on windows 7/8/10

What could be the issue of such behavior?