Hi Hun
All i can tell you is my experience.
What make of OPKs are you using?
I was using Tesco but they were always as dark so stopped using them. I then used one step (from Amazon) and when i did ov it was so obvious the line was sooo much darker there couldnt be any confusion at all.
Make sure you test between 11 am and 8 pm after not drinking since last wee (it can dilute the result) and holding yourself as long as poss (2 hours or so). Always try to test around the same time of day.
Many people test twice a day around time of expected OV so they do not miss the surge.
I also bought a clear blue ov detector so i could cross check with that, if you get a smiley face you are ov. They are expensive, but i waited till my cheap opks were dark before using them so only used 2 sticks (1 per day) out of the 7 provided, so i still have 5 left for this month.
I can also tell you I had the Mirena removed just 4 weeks ago. I had a withdrawal bleed for 4 days 2 days after removal. I ov'd 2 weeks later, and had AF start 2 days ago, so am staright back to normal. I do not believe the Mirena interfers with fertility at all. My Dr said you go back to normal straight away, and compared to other contraception methods I think you are fertile a lot lot quicker post using it.
Also try temping on Fertlity Friend xxx

xxx