[NovaScotian]

Because I've been having the same problem on a PM dual-core G5/2.3 (10.5.5) of backups not always happening, I wrote a script to check:

set tLog to alias (((path to application support from user domain) as text) & "SuperDuper!:Scheduled Copies:Smart Update ACB-500_Bkup from ACB-G5_Leopard 1.sdsp:Logs:")

tell application "Finder"
	set LogFiles to (files of tLog)
	set tLast to name of last item of LogFiles
	set tid to AppleScript's text item delimiters
	set AppleScript's text item delimiters to "-"
	set tParts to text items of tLast
	set item 3 of tParts to first word of item 3 of tParts
	set DP to items 1 thru 3 of tParts
	set AppleScript's text item delimiters to "/"
	set tDate to DP as text
	set AppleScript's text item delimiters to tid
	display alert "Last Logged Backup on " & tDate giving up after 2
end tell

The script confirms that I am occasionally missing backups. My solution is the same as the OP in this thread -- cancel the scheduled backup and recreate it. After than, things work normally for a while and then die again.