When you open up the WHS Console on one of your clients does it say 'Storage balanced on: (date and time)'.
If yes then the balancing service is running, it works once an hour IIRC.
Have you set duplication 'on' for your videos folder?
The balancing name is misleading, it does not balance the data across the discs, it just makes sure all the duplicated files are spread across different physical discs.
The WHS will place new data on the first available disc, then duplicate it as required starting at the last disc with space availble. The first disc will continue to fill until it has about 20Gbytes of free space, then it will move onto the second disc and so on. There is a Microsoft document available here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=40c6c9cc-b85f-45fe-8c5c-f103c894a5e2&DisplayLang=enthat gives the low down on how the drive extender program works.
Hope this helps.
