Is the default set at the table level or the form level. It sounds like it is at the form level. If so, is the textbox bound to a field in the table or is it unbound? Check the ControlSource is it '=Now()' or is it a field name in your table?
default value of a field in table level assigned only when a new record created. it is default value for new records. if you want to update each time a record changed, you should update it in before update event of the form. ofcourse if this is what you want to do.