By refering to Lady Macbeth's weakness I assume you are talking about Act II scene 2, when she says to herself as she is anxiously waiting for her husband to return from killing Duncan:
Had he not resembled
My father as he slept, I had done't.
Personally, I think this admission of weakness on the part of Lady Macbeth is very unexpected and not something that we would have anticipated. If you have a look at the kind of language that Lady Macbeth uses in Act I scene 5 when she willingly abandons herself to evil in order to commit the crime, and then if you consider how it is she who tells Macbeth to "act like th'innocent flower, but be the serpent under it," and then goes on to perfectly demonstrate this by welcoming Duncan with such grace and humility to her home whilst plotting to kill him, we are presented with a picture of Lady Macbeth that does not seem to have room for any weakness whatsoever.