We can conjecture that a werewolf transformation is caused by the moon as a body on the whole fully reflecting sunlight; being physically on it could mean the effects are minimized or nullified.

But lest you think werewolf fails are a phenomenon of the pre-CGI past, I call your attention to the otherwise excellent BBC series "Being Human." (Warning: there's a nekkid tush in this video ...

A new Halloween Horror Nights maze at Universal Studios Hollywood will re-create the Oscar-winning man-to-beast transformation from the 1981 film “An American Werewolf in London.” ...