Yes a circular polarizer works by blocking a certain frequency of light and it works in relation to the sun/light source. So when you might get good effects on one day with it positioned at say "15" past vertical on another day with the sun/light is coming from a different position then the polarizer will have to be rotated to a different position to get the same results. That's why they rotate
You can usually see the difference through the viewfinder while rotating it and depending on what effect you are seeking will depend on how you position the filter.