The Event Log shows the following on client that won't backup:
12:31:08 Backup set 6 started on WHS
12:31:20 Volume Shadow Copy Service sent start control
12:31:21 Volume Shadow Copy Service running
12:31:21 MS Software Shadow Copy Provider sent start control
12:31:21 MS Software Shadow Copy Provider running
12:31:21 COM+ System Application service sent start control
12:31:21 COM+ System Application service running
-- system reboots --
12:32:20 MS Windows 5.01.2600 Service Pack 3 Multiprocessor Free
The Event Log on the clients that successfully backup do not have the +COM entries but instead have "Backup of volume C: to WHS started", etc.
I hope this helps explain what is happening.