Why did Par'oh not ask his magicians to remove the frogs?
Oznayim la'Torah: Because they were not careful about the three things that protect a person from witchcraft, 1 in which case they could not witchcraft from others either.
See Chulin, 105b.
The magicians made a point of producing blood and frogs. Why did they make no effort to remove any of the the plagues?
Oznayim la'Torah: Because they were wise to do harm but not to do good by removing it.