Update your video card drivers and see if it helps.
Older drivers have been having various issues with RenPy for a while now and there are no plans to fix that, since the user is supposed to keep their video drivers up to date anyway.
p.s. When reporting a renpy VN bug - you should, at the very least, provide log.txt from the game's main directory, as well as a screenshot of the problem, in case of a visual bug. More often than not, it's very hard to help using only a short description of the problem.