Only Some IFrames Work

https://dev.smsstudios.com/citizens/education/reference-library/

For some reason, only the first iFrame loads. I am getting this error in the console: Refused to display ” in a frame because it set ‘X-Frame-Options’ to ‘sameorigin’.

All of the iFrame URL’s are from books.google.com – I’m not sure why only some of them load.