How to identify a good copy of CPL

Hi I used to own a CPL long time ago (lost it already), when I put it in front of a LCD monitor and rotated it, it couldn't completely block the light from the monitor. I have tried this on different monitors and circumstances, none of them had the good outcome. At that time I thought my CPL wasn't "high class" enough to do that, cuz when I saw some online tutorials their CPLs were able to turn completely black. And it was just yesterday I tried my friend's CPL (same model) again and surprisingly it worked! When I held it in front of my monitor and rotated it for a certain angle it somehow managed to block all the light coming from the monitor. Thus I knew I was ripped off...

Now I am planning to get another CPL this time, my question is, is the one i used to have a bad copy? good copies are generally supposed to be able to block light from a monitor?

Thanks in advance.