I tried this as well using a non-Kobo acquired commercial epub (The Lamorna Wink by Martha Grimes). Gentium, Gentium Book and Charis -- all for SIL -- render in one weight only; but all Kobo supplied fonts and others I have acquired such as PT Serif display correctly -- all weights and styles.
I would raise it with SIL; they must be doing something non-standard.
PT Serif and PT Sans are from Paratype and available under open (ie free) license. See:
http://www.paratype.com/public/. The serif ismy favourite font for the Kobo Touch.