Drobo

5N2 Won't Do Time Machine Backup

I have 2 iMacs and a 5N2 on an ethernet network. I set up iMac #1 for Time Machine backups months ago, and it has performed without a problem. However, iMac #2 will not perform a TM backup. TM says it is waiting to complete the initial backup, but nothing is happening. Why would one iMac backup work fine, and the second will not perform? Any help would be appreciated.
Thanks