Time Machine not backing up certain files

I just started a Time Machine backup on a 2 TB Western Digital external hard drive. While the initial back up went well all the subsequent backups aren't backing up new files that I have added to my machine. When it does a back up it says that it only has to back up a few KBs, even if I have added a 1 GB movie to my computer in the meantime. Then when I check the backups in Time Machine the new files aren't there. How do I fix this?