I'm sure this has been asked many times, so please forgive me for not going back over all of the previous posts. The reason I ask, is that some of my questions have a very low correct answer rate and while not wanting to make a quiz too easy, I like to give every player a chance, so I was thinking of changing a few of the more difficult and/or obscure questions. Is this permitted in editing?

You cannot just go ahead and change your quiz, apart from fixing typos and mistakes obviously. But as long as you take your quiz offline and resubmit it after you've changed a question it should be fine. Just don't forget to explain to the editors why you took the quiz offline and which question(s) you changed.

One way to make sure all editors can see your explanation would be to send a correction note to yourself on your own quiz. That note will stay attached to your quiz until it's placed back online and will be visible to all editors.

Thanks for the quick reply, Leau, one other question, do editors see this as a legitimate reason for changing a quiz, rather than writing a new one?

I suppose it would depend on the extent of the changes. If you were to add hints to the questions, or change fill in the blanks to multiple choice, I think that would be fine. If you were to completely substitute easy questions for hard ones, and did this for more than one or two questions, I think the argument could be made that a whole new quiz would be in order.
Another thing to look at is WHY your questions are hard. Is it just a difficult question, that not many people know the answer to, or is your wording a little ambiguous, so that even someone who really knows the subject can't tell just what you are getting at? Are you asking very picky date and number questions ("What is obscure novel character's address? a) 10 Elm St. b) 11 Elm St. c) 12 Elm St. d) 13 Elm St.")?
Players don't mind hard questions, it's the tricky or picky ones that get your quiz a low rating.

What Miss Agony said.
I keep getting things like this on the daily quizzes and they are almost always from older quizzes. They are asking for really obscure things because people haven't put themselves in the place of the person taking the quiz.
I'd say that is the main reason for the ratings going down.

If you're doing an essay, or taking a pencil and paper exam, or even a pub quiz out loud, it's a different story of course, but here, if you have something like Agony cites, your question will just be a random guess.