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 magic is incapable of creating anything that moves.
As the Midrash Rabah specifically writes (See Ramban DH 'u'Mah she'Yechazek'').
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. Chayim Kanievski Shlita: The entire ground was covered with lice to the depth of an Amah.