Hierarchy Nodes not adding up.

in my report, one of the heirarchy nodes is not adding up correctly with the values from its sub-devisions. For example, the field selling value for my top node should be the sum of all the field selling values of its sub-devisions (dont know if i'm using the correct term, but hope u understand). What could be the reason for this. Let me know if you need more info.

Thanks

Fran