Exodus 9:1-6 Then the LORD said to Moses, “Go to Pharaoh and say to him, ‘This is what the LORD, the God of the Hebrews, says: “Let my people go, so that they may worship me.” If you refuse to let them go and continue to hold them back, the hand of the LORD will bring a terrible plague on your livestock in the field—on your horses, donkeys and camels and on your cattle, sheep and goats. But the LORD will make a distinction between the livestock of Israel and that of Egypt, so that no animal belonging to the Israelites will die. The LORD set a time and said, “Tomorrow the LORD will do this in the land.” And the next day the LORD did it: All the livestock of the Egyptians died, but not one animal belonging to the Israelites died.
Yet in the same chapter:
13 Then the LORD said to Moses, “Get up early in the morning, confront Pharaoh and say to him, ‘This is what the LORD, the God of the Hebrews, says: Let my people go, so that they may worship me, 14 or this time I will send the full force of my plagues against you and against your officials and your people, so you may know that there is no one like me in all the earth. 15 For by now I could have stretched out my hand and struck you and your people with a plague that would have wiped you off the earth. 16 But I have raised you up [a] for this very purpose, that I might show you my power and that my name might be proclaimed in all the earth. 17 You still set yourself against my people and will not let them go. 18 Therefore, at this time tomorrow I will send the worst hailstorm that has ever fallen on Egypt, from the day it was founded till now. 19 Give an order now to bring your livestock and everything you have in the field to a place of shelter, because the hail will fall on every person and animal that has not been brought in and is still out in the field, and they will die.’”
Perhaps the use of the term "all" isn't necessarily an absolute.
Consider Exodus 7:24
And all the Egyptians dug along the Nile to get drinking water, because they could not drink the water of the river.
Does that include Egyptian infants and people too sick to dig wells?
I'd also like to mention that the word used in the two passages isn't the exact same one. Not that I know if it makes a difference.
9:6
http://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=H4735&t=KJV
12:29
http://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=H929&t=KJV