Connecting 2950 and 3550 Switch

Hi All

I was wondering if someone could tell me where I have possibly gone wrong.

I have connected 2 2950 switches together and a 3550 switch to one
of the 2950's. I have configured the following on all the switches:

Vlan 1 with ip address and no shut

Each interface connected to another switch is configured as a trunk and on the 3550 I have configured the encapsulation to be dot1q.

Now, the problem I am having is:

I can ping the 2950's from each other.
I can ping the 3550 from the 2950 it is connected to, BUT

I cannot ping the 2950 connected to the 3550, from the 3550!!!

What have I left out??

Thanks in advance

I fired up the simulator this morning and it is all working, must be a bug in Boson Net Sim.