digital clearblue advanced on timers?

Ok so hopefully this will make sense... are the clearblue advanced opk on like a "timer" . I've read before somewhere that you have to test at the same time to get result. I tested the past two mornings and have gotten the flashy "high" faces. I tried again just now to make sure I wasn't going to miss peak, but I used a different digital reader, but the same brand, and it showed me the blank O. Is it because I used a different reader that doesn't know my levels better, or did I really stop surging already? HELP!!!