Cisco to Cisco to Force10 not working

Hello,

I currently have a Cisco C3500XL switch that is trunked up to another C3500XL switch which is trunked up to a Core Catalyst 6509.  I'm attempting to replace the 6509 with a Dell Force10 S4810 switch.  When I do that, the directly connected C3500XL works, all the vlans pass traffic. However, the edge C3500XL doesn't work.  Nothing attached to that switch is available through the Force10.  I've double checked that all the ports are tagged correctly.  Is there something I'm missing here?  

On the edge and distribution C3500XLs, the uplinks are configured as such:

(from edge to distribution, and from distribution to core)

switchport trunk encapsulation dot1q
switchport mode trunk

And, on the Force10, the uplink is configured as such:

no ip address
portmode hybrid
switchport
flowcontrol rx on tx off
no shutdown

On the Force10, all of the vlans that are configured on the Cisco switches have the uplink port tagged.