How are poll taxes, literacy tests, and the grandfather clause conected to political failures?
The term "political failure" is often used to refer to an instance in which the political system fails to act in the way that it is supposed to. In such a case, the political system allows results that are contrary to the goals of that system. The things that you mention are a case of political failure.
In the time after Reconstruction, the three devices you mention were used to prevent blacks from voting in the South while still allowing whites to vote even if they were poor and/or uneducated. Allowing one race to vote while preventing another from voting is completely contrary to the values that are supposed to be enshrined in the Constitution of the United States. Therefore, when the Southern states' political systems allowed these things to be enacted, a political failure occurred. It occurred because the political system failed to create results that were congruent with the values it was supposed to uphold.