Have it ever happens that taking plan b within 72 hours but still pregnant?

Definitely: You should not depend on Plan B as primary birth control. While it works fairly well it is not as effective as using appropriate birth control in the first place. That's why it is called plan "b" !

Suspect so: It is not 100% effective - it become less effective over time. 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.