Can someone explain this to me?

I've been trying to extend my stay here until mid-August and every travel agent that I've turned to has told me that there are no seats available at all for August. Well, when I checked KLM and Northwestern (KLM's partner) websites, both have economy class tickets available for the 17th. So why can't they change my ticket? I am so peeved, I just sent a nice but somewhat direct email to my travel agent, asking her what the hell is going on! Ok, well, I didn't write "hell" but pretty much everything else. Stupid airlines, they all suck!