When a witness answers "I don't know" in a deposition, it's not necessarily your cue to move on to another topic. Say that you've just asked Mr. Jones what happened in a meeting on October 5. Mr. Jones responds that he doesn't know. If you already have a version of the October 5 meeting you want to present through other witnesses, you might be happy that Mr. Jones's claimed lack of knowledge will prevent him from giving a different account.
Or will it? At trial, Mr. Jones might remember October 5 and dodge your attempt at impeachment by saying that following the deposition, he refreshed his recollection by talking to others who were there or by reviewing documents.
It's one reason why you can't always stop at the "I don't know." Ask the witness whether he knew once and now can't remember, or whether he was never in a position to know. Follow up accordingly. If he was never in a position to know, pin him down as to why, which will probably eliminate him as a witness concerning the events of October 5. If he was once in a position to know, ask what he could do to refresh his recollection of events, then consider doing those things yourself--for example, by showing him documents or telling him what others have said.