According to the authors of your text, once participants in milgram's studies delivered the first shock to the learner, this created internal pressure to obey. this dissonance made it more difficult later for participants to

The answer to this question is draw the line as to what exactly was too strong of a shock.
After a certain amount of shock of delivered, the learners plea to stop the experiment increased according to the level of the voltage.
When the learners are encouraged that they could take more, they felt some mental reassurance that they could take more because they do not understand the thresold of other learners can do.