Yes, It is a perennial problem.
One idea is not to simply post every new puzzle in the Rookies section then for it to work its way up the other boards, unless and until it's solved, but also to post new puzzles on the Experts, Pro's and even the Masters Boards, according to their difficulty.
It could be a way round to encourage the Pro's more, if they had more of their own puzzles.
You could also actually bar Rookies and Experts from answering on the higher boards - usually they've had their chance. But on the other hand, if the puzzles are posted straight to the higher boards then the Rookies and Experts would never get their chance at them. Not much difference from the Pro's never getting their chance at many puzzles then..!