By Default the Cursor you obtain when you give a SPT command is not updatable. You will have to make it updatable using Cursorsetprop()

Set properties like KEyFieldList , Updatenamelist , UpdatableFieldLst and most important set the Sendupdates property to .t.

After giving Tableupdate() give a SQLcommit too (to commit it at the server level)
Hope this helps you

Archana