Focus on the positive. You have a job. The nursing market is saturated right now and many of us are not so fortunate. Continue with the interveiws, at least you got far enough in the process that you WERE interveiwed.....that's a good thing! Please try to not take it personally. You'll be hired by the folks who find you to be the right fit. Best of luck to you!
Perhaps look into taking a class or going to a lecture/conference related to case management. I' not in case management so I don't know what is out there but I'm sure there is something. Without experience in the area you are probably losing out to people that have case management experience. I'd look into a class or something so you could show employers you really are interested
I agree, strengthen your resume by educating yourself in those areas. Join your local specialty organizations for those fields - these are great ways to network and get access to 'insider' openings. Are there any task forces or committees in your facility that work with these areas? If so, volunteer to join them.