Disk Full Error... Won't Delete Old Backups

Hello- Since upgrading to ML, Time Machine is notifying me that disk is full and will erase old backups on next backup. Problem is, its not deleting old backups. Thus, its not backing up any longer. This has only happened since upgrading to ML this week.

The setup:

- 2012 MB Air with 250GB SSD and ~ 75GB unused

- Latest ML Updates

- External 1TB HD totally devoted to Time Machine- 23GB remaining space available

It has been setup this way since 2012 and this is the 1st time Time Machine is unable to erase old backups.

Thoughts?

Thx!