Theresep - can you post a link to the pattern? It'll help someone know how to help if we can see the instructions before and after your trouble spot. And often just seeing a pic of the finished project helps decifer any problems, too.
So this is just a guess: After you cast off the 21 stitches, are there any stitches left on your needles?
It could be that you just use the hanging working yarn to do the 12 cast on stitches, attached to the end of any stitches remaining after the 21 cast offs.