What are the chances that plan b will fail? for what reasons would it fail?

Depends: The risk of failure increases with the increase in time after intercourse before it is taken. It falls from about 95% effective within 24 hours to 85-87% effective at 72 hours.

Plan B: Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes. Major reason for failure is waiting too long to take it.