Attachments available to some, but not all ???

I've got a weird issue with a published course and I'm hoping someone here can assist me ...

I have Attachments set up, linked, and published on my course ... but only some people are able to access them.  I've sent it to about a dozen people to test and provide feedback before putting online, and about half of them are responding that when they click on the attached file links (from the Attachments link), nothing happens.  The other testers have no issues at all and the attachments open up just fine.

They're not getting any error message, just nothing happens.  Has anyone seen this before, and if so, what was the remedy?  I've walked through browser settings, etc. with them and everything looks OK.  I'm stumped!

Thanks in advance!

Heather