Why does lady Macbeth fail to answer Duncan's inquiry as to the whereabouts of the Thane of Cawdor in Act I, scene 6?

Although we don't know exactly where Macbeth actually is when Duncan arrives at Inverness and is greeted by Lady Macbeth, perhaps her avoidance of the king's question is based on her husband's behavior at the end of scene 5. She doesn't trust Macbeth to be in Duncan's presence yet because she fears Macbeth may betray their as-yet fragile murder plot merely by the expression on his face. She had only a brief time to speak with her husband upon his arrival at Inverness.

In 1.5 she warns him that his face "is as a book where men/May read strange matters." Then she instructs him to "Look like the innocent flower,/But be the serpent under 't." Macbeth's response to her encouragement indicates he hasn't made up his mind: "We will speak further" (1.5.83). He simply isn't convinced yet that killing Duncan is an option.

On the other hand, Macbeth may simply be recovering from the journey by cleaning up and resting. After all, he arrives just before Duncan and his entourage.