Microsoft forms not working properly

Hi 

I tried to set an exam on Microsoft forms today.  For some students it worked fine they did the work and it submitted.  For other students they were unable to see the images, but by sending them a separate document with those in i was able to work around that.  Sadly for a out at third of the students they were unable to submit their work and it was lost.  Does anyone have any ideas as to why or what I can do to avoid this happening again?

Thanks

Ed Johnson