Taking clomid cd3-cd7

Cara
This is my 2nd round of clomid 1st round it worked fine so we kept the same dose just took on different cds but this round I don't think its working I'm on cd18 and my opk test have gotten darker but then they got lighter and today they are starting to get alil darker again is this normal or do you guys think I'm out? Or do you think its just where I took it on different cds?