Why were the magicians unable to produce lice?
Rashi and Ramban (citing the Midrash): Because demons have no power over anything that is smaller than a barley-grain.
Ramban #1: The magicians struck the earth (like Aharon did) and said the necessary incantations to arouse the demons or did whatever they normally did to produce the lice, but it did not work (because Hashem decreed that it should fail).
Seforno: Because witchcraft is incapable of creating anything that moves.
Moshav Zekenim, Hadar Zekenim, Da'as Zekenim and Rosh: Neither could they produce lice via witchcraft (sorcery), because witchcraft only works if the magician is standing on the ground 3 (Yerushalmi Sanhedrin 6:6), and here all the earth had turned into lice. 4
Gemara, Sanhedrin, 67b. Another opinion there states that they could produce objects of any size but that they could not gather to one place objects that are smaller than a barley-grain to one place.
Hadar Zekenim: It says "Lehotzi Es ha'Kinim v'Lo Yacholu", i.e. they tried to remove the lice from under their feet in order to stand on the ground, but they could not.
R. Chaim Kanievski Shlita: The entire ground was covered with lice to the depth of an Amah.
Why does the Torah repeat "Vat'hi ha'Kinam ba'Adam u'va'Beheimah"?