For one, they don't work for everyone. There are cases (some of which are described below) in which the body has an elevated LH level due to other reasons, and an LH surge won't indicate ovulation. Additionally, ovulation test strips do not test the viability of eggs or sperm, and they don't affect fertilization.

Why do ovulation tests not work for me?

Another possible reason you won't get a positive result is you're not ovulating. It's not abnormal to have one off cycle, once in a while. However, if you don't get a positive result after a few months, or if your cycles are irregular, talk to your doctor.

Can I have a negative ovulation test but still ovulate?

Along with pregnancy tests, it is possible to get a negative result on your ovulation test when you are in fact ovulating. One thing you can do to avoid getting a false-negative test result is to be sure you are testing early or late enough in your cycle.