videos wont go into itunes?

ive added videos onto itunes before and put them on my ipod. I am trying to put a few more but for some reason when i try to add the file to the library or import them into itunes they do not show up in my library. they are the SAME type of files as the ones that are already on my ipod through itunes. any ideas?
my videos are all in AVI format and the like 6 or 7 movies that i already have imported into itunes and put on my ipod are ALSO avi format so im trying to figure out why some of the movies of the same format will not import into itunes.

i dont have the money to purchase any software