Build.prop edit doesnt stick???

decided to take a risk and add a line to my build.prop about preferred network..... but it didnt work!! I told it to use WCDMA only and it is STILL switching over to HDSPA... i get better reception when i go into the phone info menu and switch it to WCDMA , but it will not stick..... i thought adding a build prop line would finally do the trick, but its still auto switching!!

help?