The failure of Bnai Yisrael to listen to Moshe initially weakened his strength as a leader. A leader, says the Sefas Emes, draws his inspiration and strength from the people. If they do not wish to be redeemed, in whose name can Moshe demand redemption? Moshe therefore complains, "I have uncircumcised lips," meaning, "I can't express myself properly" (שמות ו:י"ב); he is frustrated at the people's failure to respond to him, and he thinks they don't really want to leave. When Paroh confronts Moshe and Aharon asking why he should set Bnai Yisrael free, he is really saying that Bnai Yisrael must first have the desire to leave! Usually, rebels will fight when seeking liberty. These people, however, have shown no desire to leave, and Paroh does not want to let them go. The Chasam Sofer adds that Paroh is right; Bnai Yisrael were low spirited and afraid of a war with Egypt. Therefore, nothing could be gained by telling Bnai Yisrael to leave without Paroh sending them out, because they would not go. Moshe had this in mind when he said to Hashem, "Behold Bnai Yisrael don't listen me," meaning that they show no desire to leave, "so why should Paroh listen to me", when I request that he set them free (שם)?
Rashi, (שם בד"ה ואיך) quoting the Midrash, declares Moshe's argument to be one of the ten examples of a Kal VaChomer, a logical deduction, found in the Torah. If Bnai Yisrael were not willing to listen, then why should Paroh? But many Meforshim question the validity of this argument as a Kal VaChomer. Earlier, the Torah attributes Bnai Yisrael's failure to heed Moshe to their impatience(שם). Since this impatience was not a problem for Paroh, the Kal VaChomer is insupportable.
The Daas Zekeinim MiBaalei HaTosafos, however, explains that Moshe, by saying the words "I have uncircumcised lips," meaning, "I am slow of speech," reinforces the Kal VaChomer. As far as Paroh is concerned, Moshe's speech defect represented an insult to the throne. Hence, Moshe was convinced that if for any reason Bnai Yisrael would listen, surely Paroh would not. The Kesav Sofer adds that Moshe was heavy of speech and tongue, that is, he stuttered and did not know the language of Egypt. With regard to Bnai Yisrael, he had only one shortcoming, as he spoke their language. But with Paroh, both problems presented themselves. This is the meaning of Moshe's claim "And how then shall Paroh hear me (as I do not know his language), and I am of uncircumcised lips (being also slow of speech and tongue)" (שם).
Interestingly, Rashi (שם) explains the end of the Posuk (שם), that is, the phrase "And I am of uncircumcised lips" before explaining the phrase "why should Paroh listen" which precedes it. The Levush HaOrah interprets Rashi's intent in accordance with what was said earlier, that the Kal VaChomer was based on Moshe's claim of a speech defect. Moshe is thus saying that Bnai Yisrael, whom I addressed in their mother tongue, did not listen to me because of my speech defect; Paroh, when I will address him in a language foreign to him, surely will not heed me. This Kal VaChomer cannot be contested, for we can't dispute human nature. The "impatience" shown by Bnai Yisrael, according to the Levush HaOrah, was a consequence of Moshe's speech defect. They became impatient with him when they found it difficult to understand him, and he therefore concluded that they weren't really interested in leaving.