When Moses first encounters God he is given the ability to turn his hand leprous and back as a sign (Ex 4:6, 7),like turning his staff into a snake (Ex 4:3, 4), that he can prove to Pharaoh that he comes from the God of the children of Israel. We never see Moses use this ability again, through ten plagues and after. Why?
The implicit assumption in the question is that Moses was able to perform these miracles whenever he wanted. This is not true.
In fact, Moses only ever performs such supernatural miracles at the specific instruction of God.
- In Ex 4:3, 4, Moses has his rod turn into a snake and back again, but only when God told him to do so. (He never did this again although Aaron did.)
- In Ex 4:6, 7 Moses' hand is tuned leprous when instructs that it be done. (He never did it again.)
- In Ex 7:14-24 Moses tuns the Nile and water into undrinkable blood but only at the explicit direction of God (V15, 20). He never does this miracle again.
- In Ex 8:5, 6 Moses makes frog come up on the land, but only at the specific direction of God (V5) and never preformed such a miracle again.
... same with the gnats, flies, livestock, boils, etc, etc. Even the miracle of parting the red sea (Ex 14:16-21) was done at the explicit direction of God and never performed again.
Moses was supposed to deliver God's message to Pharaoh with miraculous signs for him to believe that indeed the creator of heaven and earth is at work. Mosses only did what God told him to do. Failure would be a rebellion against God.
However, he once failed by taking God's glory to himself(and Aaron) after striking the rock to bring out water for the Israelites to drink.
"... Hear now, you rebels! Shall we bring water out of this rock for you?" (WEB Numbers 20:10).
Numbers 20:8 makes it clear that God instructed Mosses to speak to the rock but mosses struck it.
Because of this disobedience God did not allow Mosses step on the promised land. He paid that with his life.
The leprous hand was the first indication of God's wonderful works in front of Pharaoh.
"Yahweh said furthermore to him, “Now put your hand inside your cloak.” He put his hand inside his cloak, and when he took it out, behold, his hand was leprous, as white as snow. He said, “Put your hand inside your cloak again.” He put his hand inside his cloak again, and when he took it out of his cloak, behold, it had turned again as his other flesh" (WEB Exodus 4:6-7).
God gave Mosses particular instructions to follow. From verses 8 - 9 we can see that God placed a conditional clause there by using "if they will not believe you..." followed by what Mosses should do next.
All the plagues and other signs and wonders came from God's command at the appropriate moments.
And Yes we never saw Mosses perform those miracles again because God did not instruct him to do that again.
The signs of turning the staff into a snake (nachash) and turning the hand leprous as snow were not signs for Pharaoh but for the Israelite elders (see Ex 3:16–18 and 4:28–31). Moses' staff does not turn into a snake (nachash) before Pharaoh. Instead, Aaron's staff turns into a crocodile/serpent/dragon (tannin) before Pharaoh (Ex 7:9–10).
The two signs served their function when the elders believed Moses (Ex 4:28–31). There would be no need for Moses to perform these signs again.
Nevertheless, they do show up again in a different manner.
As Richard Elliot Friedman points out in his Commentary on the Torah, the signs of turning the staff into a snake and turning the hand leprous as snow foreshadows the events of Moses making a bronze snake (nachash) on a pole (Num 21:5-9) and Miriam becoming leprous as snow (Num 12:10).
The Hebrew word "Metsorat" (מְצֹרַ֥עַת) in Shemot (Exodus) 4:6 means "Abscessed" pertaining to skin burns / infections. * Moses (Moshe, משֶׁ֣ה) is defended by the “Anger of YHVH” (אַ֧ף יְהֹוָ֛ה) which uses the Metsorat "מְצֹרַ֥עַת" leprous plague as a punishment against his defiant sister Miriam in Bamidbar (Numbers) 12:10.