OPKs don't work for everyone. I used to test 3x a day and have never seen the likes of a + OPK on a natural cycle.
This. Two different REs told me that some women just don't get enough surge or it doesn't get picked up.
You could also be over-hydrating and diluting the surge.
Try taking them 2-3 times a day around your projected cycle to see if you just have a short surge. Also - it is recommended to take them around noon, not when you wake up, because that's when it builds up to peak amounts.
FWIW - I have never gotten a + OPK, but I also don't ovulate on my own. Even when I make big follies, my body will just hold on to them until they form a big cyst. I have to trigger to release my egg(s). HTH