These students are most likely using Google Chrome, which doesn't enable Flash by default for security reasons.
This is how to turn it on for a particular website:
Use or fix Flash audio & video - Computer - Google Chrome Help
I'll add to @gunther 's advice with the fact that student (and you) must look out for prompts from the browser. In fact, I've had the best experience using Chrome with Conferences as opposed to other browsers because BBB introduced a feature a few years ago with Chrome and Firefox users by having them specifically enable the software via a few dialog boxes that crop up, as outlined here from the student's perspective: How do I join a conference in a course as a student? As that page indicates, Flash is only used for IE and Safari users. If it still does not work, I have usually just recommended the use of a different browser, but Chrome most definitely should be working....as should Firefox....without worrying too much about Flash. There's good reasons that Flash is on its way out; it's only a matter of time (we hope!) before IE and Safari join the parade.