Ovulation test.. line getting darker?

I have anywhere from 32-38 day cycles but usually 32-34. I got my Mirena iud removed in August and we've been TTC ever since with no luck so this month I started using the clearblue advanced digital ovulation tests. It said to start testing on day 12, but I forgot and thought it said day 15 so I started on day 15, lol. Yesterday and today have both been "low fertility." Is it possible I missed the day of ovulation since I accidentally started a couple days later than what it says to? Is the line getting darker? I just don't want to use any more tests this month if it's a waste and this is my first month using opk's so I'm new to this! I'd appreciate any insight. Thanks!