Yes, some programs are competitive to get as away/guest rotations. There are several reasons for this. One, the program has an expectation that you will be at a certain "level" when you walk in the door. Your month with them could be a nightmare for both you and the attending if you get there and can't work at their level. Programs use board scores and grades to determine "where you are". Two, those programs get applications from all over the world, and there is no way to take every student that applies.
Advice? Fill out the application in its entirety and then proof it. Make sure that it looks good and says something compelling (if you need to write an essay). If you have any interest in their program for residency, be sure and say so. And, if the program needs a letter of good standing from your school and you can swing it,try to get a letter that highlights you and your clerkship work (i.e., positive comments from your preceptors, etc).