But wherefore could not I pronounce “Amen”?
I had most need of blessing, and “Amen”
Stuck in my throat.

These lines provide an excellent example of enjambment, a word defined in one dictionary as "The continuation of a sentence from one line or couplet of a poem to the next. Much English poetry, including Shakespeare's, is written in iambic pentameter because it was generally assumed that ten syllables was about as much as a person could speak without pausing for a breath. With enjambment, as in the last two lines quoted above, there is no logical place to pause, and it is as if there is only one long line reading: "I had most need of blessing, and 'Amen' stuck in my throat." The enjambment invites, or requires, the actor to speak the words "Stuck in my throat" breathlessly, or hoarsely, as if the word "Amen" actually does stick in his throat. An actor might actually inhale as he speaks the words "Stuck in my throat." This vocal effect is usually the purpose of enjambment. As far as what the words mean, Shakespeare probably only wanted a vocal effect to enhance the horror of the aftermath of Duncan's murder. Macbeth is still trying to say the simple word "Amen" in the hope of obtaining blessing, but he still can only manage to croak it. It could be said that his inability to say "Amen" shows that he is eternally damned. Once he has accepted his damnation, he becomes a monster of tyranny because he has nothing to lose. 

Here is an example of enjambment from Othello:

O, balmy breath, that dost almost persuade
Justice to break her sword!

Here Othello uses up all his breath in speaking the first line and must choke on the word "Justice" and the words that follow it. He would seem to be gasping for breath because he is suppressing his mixed emotions of jealousy, grief, adoration, and love, as well as his terrible inner conflict arising from wanting to kill Desdemona and wanting to let her live. These two lines are a somewhat better instance of enjambment than the foregoing lines from Macbeth ending with "Stuck in my throat." The enjambment in Othello's lines is more conspicuous because the first of the two lines is crafted in such a way that the actor will exhale a great deal of his breath with "O," "balmy," "dost," "almost," and "persuade," and will be literally breathless when he comes, without pause, to the word "Justice." A strong actor might speak the entire line while inhaling, whereas a weak actor might pause to inhale after the word "persuade" and then come down much to forcefully on the word "Justice" at the beginning of the next line.

Here is an example of enjambment from one of Wordsworth's sonnets:

The holy time is quiet as a Nun

Breathless with adoration

Macbeth couldn't say "Amen" when he heard the men wake in the night, and he feels he needs to say it (to pray, and receive forgiveness and blessing) now more than ever, since he just murdered innocent men in cold blood.