An Ovulation Predictor Kit is, in general, a useful way to determine when ovulation will occur. An OPK checks for lutenizing hormone (LH) which peaks just before ovulation and prompts the release of the egg.
As with any home test, an OPK is not perfect. For example, you may catch the very end of the LH surge, and be too late to conceive. Some women have generally high LH levels, making it difficult to know when you are truly ovulating. Sometimes polycystic ovary syndrome (PCOS) can cause elevated LH levels, and a woman with PCOS may or may not actually ovulate.
You might also get a positive OPK result and not ovulate under other circumstances. For example, your body can prepare to ovulate, leading to an LH surge, and then your body fails to release an egg due to illness, stress or some other cause. Once the egg fails to release, your LH surge will drop and youer cervical mucus will dry up. Your body may try to ovulate again once the stress factor is resolved, or you may not ovulate again until your next cycle. It is a good idea to chart other signs of ovulation, such as basal body temperature and cervical mucus to confirm that the LH surge does indeed indicate ovulation.
Finally, Clomid may cause false positives to occur. Check with your physician to see when a reliable OPK can take place if you have been taking Clomid or other fertility medications.