Ex 6:12 And Moses spoke before the LORD, saying, "The children of Israel have not heeded me. How then shall Pharaoh heed me, for I am of uncircumcised lips?"

13 Then the LORD spoke to Moses and Aaron, and gave them a command for the children of Israel and for Pharaoh king of Egypt, to bring the children of Israel out of the land of Egypt.

Moses became painfully aware of his inadequacy. He admitted that he was not a great speaker as if great oratorical skills could turn things around. 

But Moses was the right man for the job. And so God charged both Moses and Aaron to give it one more go. 

We put so much premium on our abilities. If it all depended on us like what we can do and all then the glory would be on us. If Moses was able to move Pharaoh and the children of Israel through his great speaking ability then people would say "It's because of his great speaking abilities! Wow! Amazing!"

But, if Moses had a weakness like what he had and the whole thing was done, people would say: "It's obviously not Moses' ability. It's God! Amazing how He can use a weak instrument like that!" All glory would go to God.