Don't rely on the heat of dilution to provide enough energy to oxidise all the carbon.  Use a temperature regulated hot plate or block at 135 C for 30 minutes.  You will get very consistent results with this extra heating time.  Also, I always used a colorimetric determination, focusing on the 625 nm wavelength.

thank you mr Tongway

Dr Tongway- the Ferroin colorimetric endpoint for WB titration is very satisfactory for routine organic carbon, I am curious if you feel you need to add the colorimetry of 625, and how?

I used an auto-analyser to do all the colorimetric determinations, freeing up the chemist to do other things.  We used to do batches of 175 samples a day.  I am now retired and not doing any lab work.  Regards.