Originally posted by SkitaleC: Warehouses have a higher priority for export than delivery for processing ?!
Not quite. The way things work is that processing buildings send out a request for resources. Warehouses can then respond - if they have trucks available.
Meanwhile warehouses will aim to stay at a level according to their settings. If they're too full they will export, if they're too empty they will import (if possible). I recommend you read the guide I linked. I know it's a bit long, but it explains how everything works, and why having enough storage is so important. :)