The PIX won't route a packet back out the same interface it came in on, that includes packets coming in over one spoke tunnel and going back out over another spoke tunnel. No way around it with the PIX as a hub.
Fully meshing them would work, but get's messy and your workload increases exponentially as the number of spoke sites increases.
Changing the hub PIX with an IOS router or a VPN3000 would resolve your issues also.
Sorry I don't have a better solution for you.