WHere is the summary field displayed? in the header, footer or a column in the table?
You shouldn't have to redefine the field just to get it to update. You may have to click out of the field and click an empty part of the layout to commit the changed value so that the summary field can effectively recalculate the new total.
A script trigger set on the quantity field could be used to use Commit Record to force the field to update when you exit this field if it is really needed.