When I get "jailbreak failed" I just click on the jailbreak button again until it's successful. No need to shutdown or reboot in most cases, unless the console shuts itself off.
I've been having great success with Leeful v.6 on my ESP, between 90 and 100%. Now, it doesn't work anymore. Failed 7-8 times in a row. Any suggestions? Reflash my ESP with version 6 or with a newer one?