Saving a document clears all of the undo history; you will not be able to undo any action that happened prior to the most recent save operation.
Select Save (or Save All) from the Item menu; or,
Select Lock Object from the Edit menu (or press the F2 key); or,
Lock the document form using the Lock selection in the form's statusbar; or,

Close the document window, which saves all changes automatically.
1073-20100516.1311