apostrophes turn to squares

Just wondering if anyone's had the same issue, and if there's a fix anywhere.

I've a few books that I've downloaded and put into Calibre - and they look just fine when I view them. However, after they've been transferred to my Glo, all the apostrophes and quotation marks have turned to squares. It's very offputting to try to read.
I've tried reformatting them from epub to epub again - and reinstalling on the Glo - but no luck.
Any thoughts?