Opk tests strips

Dee
Ok so I'm on cd 13 and last month I used a clearblue digital opk. I got flashing smileys for 7 days before a solid smiley on cd 18. They are too expensive so figured now I know I ovulate, to get the cheap amazon ones -the strip test. I am testing twice per day and barely even getting a line. Like, no lh against the test line. Is this normal?