Opk question

So..is it normal for opks to go back & forth (dark, light, dark..) until finally getting the peak? I started testing on CD8, got a pretty good line..obviously not anything wild, it was only CD8. I tested again at 10, 11, 12 & now today at CD13. The lines have been so inconsistent throughout the days..even showing a faint line today. I did run out of the one brand I was using, and had to pick up where I left off with a different brand. Did this happen to anyone else before finally getting their peak? (I usually ovulate on day 14 for reference)