ladycplum beat me to it, but the fact that Ron doesn't come across as funny on screen can't be blamed on the actor. If he's hardly seen, it doesn't mean he needs to improve, it's because he's not given as much to do. The problem is 100% with the script. Kloves is, in my opinion, quite spectacuarly average in his abilities. I can't believe a franchise of such importance has been left in his hands. Indeed, why is Yates finishing this off, when Guillermo del Toro expressed an interest before the Hobbit? Why did Terry Gilliam not get a look in? Because Yates/Kloves are cheaper to hire and will accept studio interference.

I said in another thread that Harry's "But I am the chosen one", as done to death in the trailers, is typical of Kloves' painfully unamusing and immature idea of funny. We're not talking Groucho Marx here. Rupert is maybe the most natural, unmannered performer of the trio, but has been sabotaged from the start. First by being encouraged to mug like a fool (or Macaulay Culkin, thanks to Chris Columbus) in Chamber of Secrets when he was fine in Philosopher's Stone, to constantly having 90% of the lines that make him noble, loyal and instinctive taken away and given to Hermione, through to being shoved aside completely.

He's not the only one, Mrs Weasley, Hagrid and Ginny have also been fleeced in this way, but Hagrid's character is strong enough to miss a few lines and the others are firmly cameo performances. Ron has been the most wronged, yet has to stay far more centre stage with only the worst custard-pie level humour to fall back on. Riddikulus! Kloves says Hermione is his favourite character in the book, yet writes her totally differently. What he really likes is the idea of a fiesty teen girl, not the character we see in canon, so has changed her to fit. PoA onwards she became a super-plot device, and far less attractive, imo, than the deeper, more realistic book version.

Steve Kloves has to take a lot of the blame for any flaws in these films. I understand that more and more has to be dropped to fit the increasingly large books into one film. (If it has to be only one film) However intelligent re-writing is essential to tie up loose ends and make the resulting story make sense. He hasn't managed it since Chamber of Secrets which was a less sophisticated tale. In fairness he didn't write the script for Order which wasn't much better, but at least the characters there were a bit more on track. Sabotaging characters doesn't shorten a film it just panders to what Hollywood think 7 year olds can understand. Intelligent sub-plots that need concentration? Memories that help illustrate the nature of the main baddie? No can do. Lurve and romance and teen angst? Yes! Double it! They patronise children (and dismiss the more mature viewer) in a way JKR would never dream of doing.

Heyman? He was the one constant as the Directors changed. WB are in charge here, but he's apparently done nothing to try to control the script writer, constant changing appearance of the world or the strange acting choices.