I am not 100% sure but I wanted to let you know what happened with me... I had used Implanon for 6 years (2 rods) and when the last one came out I was diagnosed with PCOS a month later. I was told by my GP and Gynae that OPKs would not work for me because of the hormonal imbalances.
SNAP! same for me... but I was anovulatory for 8 months before getting a diagnosis. Needed clomid to concieve. Then the OPK's worked.

But anyway... there are so many varied stories on the effects of implanon. Some women fall pg immediately after it comes out and some take ages and most are just anywhere in between. But yes it could be the implanon taking a while to get out of your system.