I think this is a usability/motivation issue.
You can't vote up questions on the 'index' pages like you can on reddit/Digg/so on, so people aren't going to go 'that looks cool' and vote it up (or vice versa) before reading it.
IMHO this is a good thing, but...
As I see it, the 'workflow' of reading a question/answer goes like this:
User opens page
User reads question
Unless the question is abnormally good or bad, or otherwise provocative, this isn't likely to elicit any emotional response. It's just a question, carry on.
User scrolls down and begins reading answers
As there are many answers, and good answers are rewarded by being 'accepted' and also with increased reputation, this puts the user in the mindset of 'make the answers better'
The emotional response behind having your answer accepted or upvoted is "I know stuff, I'm smart, I feel good." Likewise, conferring that reward on someone else is quite a powerful thing too. This provides a very strong motivation to rank and provide answers.
Because of this motivation, people will put a lot of effort into writing answers (like me with this diatribe) and ranking them.
This works very well for providing and filtering good answers, but there's no such motivation behind voting for questions. For most questions, the strongest response they are likely to elicit is "I have that problem too", which while it's strong, is only going to apply to a small portion of the viewers/answerers.
While I think this is why questions aren't being voted on as much, I don't think you need to go all out to provide more motivation for it, as this would distract from the main goal of writing/ranking the answers. A simple 'nudge' to remind people to vote on questions I think will do the job without any/many adverse effects.
My suggestion for this is simply to make the voting buttons on the question proportionately larger (or make the ones by answers smaller), and possibly change the color or something.
This will draw the reader's attention to them, and send the message 'hey, while you're here, vote on the question before carrying on reading the answers'
