Gemara

Question: What is the source that Semichah is not Me'akev (regarding Korbanos that apply to all generations)?

Answer (Beraisa) Question: "V'Somach...v'Nirtzah" - atonement does not come through Semichah, rather, through Zerikah - "Ki ha'Dam Hu ba'Nefesh Yechaper"!

Answer: This teaches that if one treated Semichah like a remnant of a Mitzvah (i.e. omitted it), the Torah considers it as if he did not atone, but he did atone.

Zevachim 6a: If one treated omitted Semichah it is as if he did not atone and atoned;

He atoned for transgressions before separating the Korban, but not for a transgression after separation, i.e. not doing Semichah

Rishonim

Rashi (DH k'Ilu): If one omitted Semichah he did not do the ideal Mitzvah, but he was Mechaper.

Rambam (Hilchos Ma'aseh ha'Korbanos 3:12): Semichah is a remnant of a Mitzvah. If one omitted it, it is not Me'akev; the Korban was Mechaper. Nevertheless, the Torah considers it as if he did not atone.

Aruch ha'Shulchan Ha'Asid (Kodshim 68:19): The Rambam holds that he was Mechaper, i.e. he need not bring another Korban; it as if he was not Mechaper, i.e. he does not get reward for bringing a Korban.

Acharonim

Question: The Gemara says that it is as if he did not atone, but in Zevachim it concludes that in truth it did not atone for not doing Semichah!

Answer (Chazon Ish Nega'im 12:17 DH Zevachim): If he omitted Semichah b'Mezid, it is as if the Korban did not atone at all