I don't know about that, but I don't really think there's supposed to be chemistry between them. Lavender and Ron share one of those awkward teenage romances.
Although I guess there has to be SOME sort of puppy love kind of chemistry between them, and if she's a good enough actor, I think she can pull it off.
I'm trying to be extremely objective about this, but trust me, I am going to have an extremely difficult time watching her kiss rupert without fulfilling the urge to throw a hatchet at the movie screen.