I am having the same issue, but intermittently. Some courses will download just fine, others usually fine, some virtually never (though they did download before). They are usually relatively small courses, some without even images or audio.
@JBorrego: could you have a look at my course that doesn’t download?
BTW: I see that you found some ‘corrupt assests’ with the courses you looked at. Do you think that Memrise would be able to find why those corruptions actually come into existence and fix the root course? Thanks anyway.
Actually, that is not what I experience. When I remove the download of course, the course will ‘refresh’ itself (sometimes after a few seconds, sometimes after many tries). Downloaded courses won’t refresh. But anyway, I see it as a bug, too.