can't undo continuely in vi editor

Tue Dec 22 14:31:11 UTC 2009

when i use 'u' to undo the deleted string, i can just undo one character,
and there is  error msgs on bottom:
1 change; before #39  17 seconds ago
or
1 change; after #39  22:24:23

can advice?
thanks.
-- 
PHILIP Wang
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