Well, professional liability isn't much of a moneymaker, from the insurance company's point of view. Luckily for you, dental malpractice is a lot less hazardous, however, unluckily for you, most insurance companies won't offer just ONE class of malpractice insurance.
That means, in any given state, there is very, very little competition for professional liability coverage. You don't say where you are, but there are probably less than five carriers in your state. Several of those would probably be non-admitted carriers.
The best thing to do is see if you can hook into a state wide professional group, and have the group negotiate a 5% cut in rate based on volume discount.