I've worked in places where most people are certified, and places where most people aren't. These are the common threads I've seen in places where people get certified and maintain that certification.
1. The hospital pays for certification, regardless of whether or not you pass (staff, even those who will likely pass, may not think it's worth risking their $300 application fee if they're concerned that they won't pass and then won't get reimbursed).
2. The hospital pays for recertification fees (otherwise staff may not be willing to spend their own money to recertify, especially if they don't get any financial incentive to stay certified, which brings me to...)
3. The hospital provides some financial bonus, i.e. hourly diff, annual bonus, higher 'step' on pay scale, clinical ladder 'rung,' or one-time bonus (longer-acting incentives are more effective than one-time bonuses, especially if you want people to stay certified)
4. The hospital provides some funding or educational opportunities to help cover the cost of renewal CEUs, i.e. AACN membership (especially because the CCRN requires so many hours)
The CCRN is expensive to take and to maintain (both due to recertification fees and the large number of CEUs). In addition to the expense, employees have to think it's worth their time and effort to become certified; most people won't be willing to work their butts off and spend $100+ a year to become and remain certified (unless they're applying to other jobs and/or grad programs). From what I've seen, the combination of these elements will probably help to increase your numbers, however it's pretty pricey for the unit.