In Macbeth, how do the witches contribute further to Macbeth's downfall when he revisits them in scene 16?
I assume you are reffering to Act IV, scene i when Macbeth seeks out the witches to get advice and find out more of his fate. The significance here is that Macbeth is seeking the witches out this time. He is taking control of his own terrible fate and the witches acknowledge this, saying "By the pricking of my thumbs, / Something wicked this way comes." Instead of hapless victim, Macbeth is an instrument in his own downfall.
The prophecies of the witches both encourage Macbeth, since he feels that they grant him immortality, but they are paradoxically troubling, too. After all, if cannot be defeated--since every man is born of a woman and forests grow very slowly--why would Banquo's sons be shown as kings? Macbeth chooses to select the information that benefits him and ignore (as much as he can) the rest.
The bottom line here is that it is Macbeth making the decisions. He does not appear to be under the witches' power or spell here and Lady Macbeth is nowhere to be found. This is Macbeth choosing his fate.
The witches had started Macbeth's downfall in Act I when they prophesied that he would be the king. This made him decide to kill Duncan. Other of their prophecies made him try to kill Banquo and his sons.
In the scene you mention, the witches do more to help cause his downfall. They tell him a number of things that make him think he can never be defeated. They also tell him to beware of Macduff. Because of these prophecies, he acts in ways that make people hate him more (like when he has Macduff's family killed).
Because of this, and because he doesn't understand their prophecies completely, Macbeth ends up getting killed.
One of the main ways that this visit contributes to Macbeth downfall is that he takes what they predict: No man can kill you unless they are born of unnatural childbirth, and the Woods or Forest of Barman would come to him literally. When Macbeth sees these two things happen he would know that he would lose his throne. He thought these two prophesies would never happen because they were so unbelievable. He took this to mean he would be king a long time. Even when the forest comes to him, he thinks the prophesies about the child birth would never come true. So he never took inconsideration that they might mean more than one thing. Macbeth only thought the witches were reinforcing the first prophesies that he would be king. Macbeth let what he wanted to come true rule him, instead of using them as a warning to maybe do something about it. He let his emotions and the lusts for power rule him. This is what Shakespeare was trying to point out. He was trying to make people see how the present rulers of the time, might not have their subjects best interest at heart.