Hi Neil,
Are you using the Print Preview button on the New / Edit Estimate screen, which generates a PDF? Has this screen been customized in any way? When you click the Print Preivew button this should save any changes you have made, before the PDF is generated.
If you are referring to the total on the grid on the screen (which contains line items), this can be udpated by clicking the Update button at the bottom of the grid.
-Russell