Hi Min,

I'm another one who never had much luck with OPKs either. Mine would never get as dark as the control line, even when I was having tracking with BT's and knew full well that my LH was surging But what I did find was that it helped to compare the darkness of the lines from day to day - I would usually get an OPK on one day that was darker than the rest (but still not as dark as the control), and that coincided with a temp rise the next day confirming that O had occurred.

I HTH and good luck!