I can view, they can't

I just published something to Articulate Online which contained a video.  It works perfectly when I go in to view it but when some of my learners go in they don't get the video.  I have tried it both as a MP4 and I converted it to a Flash.  The MP4 does not work at all for them (I thought MP4 was compatible).  Again, it worked for me, but not for the learners.  The Flash one worked, but it is smaller and if I make it bigger I lose clarity.  Any advice?

Thank you,

Sheri