Launch Console from Utilities and use backupd as a filter in the top box.
A normal backup looks like this:
07/08/2011 12:33:40.364 com.apple.backupd: Starting standard backup
07/08/2011 12:33:40.945 com.apple.backupd: Attempting to mount network destination URL: afp://Colin%20Robinson@Colin-Robinsons-Time-Capsule.local/Colin%20Robinson's%20 Time%20Caps
07/08/2011 12:33:50.577 com.apple.backupd: Mounted network destination at mountpoint: /Volumes/Colin Robinson's Time Caps using URL: afp://Colin%20Robinson@Colin-Robinsons-Time-Capsule.local/Colin%20Robinson's%20 Time%20Caps
07/08/2011 12:33:55.305 com.apple.backupd: QUICKCHECK ONLY; FILESYSTEM CLEAN
07/08/2011 12:33:58.295 com.apple.backupd: Disk image /Volumes/Colin Robinson's Time Caps/Colin Robinson’s iMac.sparsebundle mounted at: /Volumes/Time Machine Backups
07/08/2011 12:33:58.623 com.apple.backupd: Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
07/08/2011 12:34:38.602 com.apple.backupd: 11.62 GB required (including padding), 86.23 GB available
07/08/2011 12:37:47.736 com.apple.backupd: Copied 1957 files (956.7 MB) from volume Macintosh HD.
07/08/2011 12:37:49.382 com.apple.backupd: 1.28 GB required (including padding), 86.23 GB available
07/08/2011 12:38:11.889 com.apple.backupd: Copied 969 files (27.5 MB) from volume Macintosh HD.
07/08/2011 12:38:19.303 com.apple.backupd: Starting post-backup thinning
07/08/2011 12:38:41.631 com.apple.backupd: Deleted /Volumes/Time Machine Backups/Backups.backupdb/Colin Robinson’s iMac/2011-08-06-113534 (1.05 GB)
07/08/2011 12:38:41.631 com.apple.backupd: Post-back up thinning complete: 1 expired backups removed
07/08/2011 12:38:41.996 com.apple.backupd: Backup completed successfully.
07/08/2011 12:38:46.238 com.apple.backupd: Ejected Time Machine disk image.
07/08/2011 12:38:48.395 com.apple.backupd: Ejected Time Machine network volume.
You may be able to spot something unusual that might help others answer your problem.
Regards,
Colin R.