Many times it happens that we download all the pieces in multiple rar files and one of the part is damaged or missing.There is no way to extract all remaining parts without that missing portion. But if it is any media file inside then you can try the trick given below.

First extract part1.rar. Suppose it is from part1.rar to part9.rar and part6.rar is missing then copy part1.rar and rename it to part6.rar. Now extract the rar files. Your file will be extracted and now you can play the file, it will play part1 again in place of part6 while playing. But if you want to see the remaining portion you have to do this.