ovulation tests and clomid

An
First clomid cycle! I always have 2 lines on my ovulation test the whole month and they gradually get darker. today is CD 15 (6 days after last clomid ) and i still have NO second line. i started opk testing on cd 12 and have been testing three times a day. does anyone else ever not have a second line and it just shows up all of a sudden ??