The likely cause is when number of images or files in their respective Object Libraries are too large. Any unneeded files or images will need to be deleted to reduce the size of the Library allowing the Pushpage Issue to copy. When you copy a pushpage issue, all content from the previous issue carries over to the copy of that issue. While this can be a time saver, over a period of time, the issues take longer and longer to copy and eventually may not copy at all.

To remove files or images from the Image or File Library:
  1. Create or Edit into a Pushpage Issue
  2. Edit a region
  3. From the Insert drop down, select either File or Image
  4. Select the Delete trashcan icon to delete the file or image

Another suggestion is to only copy an Issue around 4 times. Then, create a new issue from the template so you are starting with a clean issue.