If a home ovulation test says you are ovulating, is the Plan B pill not likely to prevent pregnancy?

Incorrect. Plan b does not prevent ovulation. It actually works by preventing implantation of an egg onto the uterus. Keep in mind that nothing is 100% effective, and that Plan B works best if the directions are followed exactly.
It should work. The Plan B regimen is designed to prevent implantation of a fertilized ovum. If you don't ovulate you don't need it.

Related Questions

What can prevent pregnancy after this mishap? : I had unprotected sex and used a plan B pill. Though ovulation was set for the day after I took it.

Plan B is designed. To work in a pinch when the risk of conception is good. It sounds like you had unprotected intercourse within 2 days of suspected ovulation date, which is within the danger window. Monitor your hormonal response in the next 2 weeks, as you should have some bleeding associated with the Plan B if it was successful. If no bleeding after 2 weeks, check 2 pregnancy tests 1 week apart.