In Episode 29, the NPA tries to capture/kill Mello. Soichiro Yagami gets the Shinigami Eyes and tells Light that Mello's real name is Mihael Keehl. So, Mello escapes because Soichiro Yagami couldn't kill him. But didn't Light know Mello's face?
It's mentioned that no matter how exact the drawings are, you can't see the names of the person with the Shinigami Eyes. But Light knew Mello's name was Mihael Keehl, he also knew how he looked like because of that girl who draws Mello's and Near's faces.
Also, one of the first rules of the death note states:
This note will not take effect unless the writer has the person's face in their mind when writing his/her name. Therefore, people sharing the same name will not be affected.
Light knew how Mello looked like. He also knew his name was Mihael Keehl.
Why didn't Light kill Mello?