Problem backing up with Time Machine

Time Machine has been runnng flawlessly to back up my MacBook Pro (OS X 10.6.8) for a number of years. I'm now getting the following error message:

"This backup is too large for the backup disk. The backup requires 12.05 GB but only 10.86 GB are available. Time Machine needs work space on the backup disk, in addition to the space required to store backups...."

Total storage used on my MacBook Pro's internal HD is 131 GB. Total capacity of the external HD I've been using for my backups is 299 GB (both these stats from "Get Info"). In Time Machine prefs I've added four items totalling 17 GB to the "Exclude from backup" list. So a full backup should be about 114 GB and incrementals should be far less than that. I have been backing up twice per week successfully for about 3 years. When I "Enter Time Machine" I see there is only one previous date backed up there.

My understanding is that Time Machine is supposed to delete older incremental backups to make room for new ones as needed. I should have many incremental backups when I Enter Time Machine, but there's only the one. How do I get Time Machine working as it's supposed to?