Edited existing group policy but machine not updating it ?

Hi, 

We have existing group policy, but every time I edited existing policy and add in new stuffs, and I would go to client machine to do gpupdate /froce, and it will not pull down the new changes, only until I put the changes into a new GPO, then it will take effect, is there such thing as machine not reading new changes to existing GPO ?  Also, our win7 machine works fine with GPO, but XP machine isn't, any clues ? Thanks!