1 x 3550 works in stack, 2 don't. Why? 1

[trismegistus]

Hi

I have 2 x cat 3524's in a stack. When I add a 3550, no problem. When I add another, the 3524's stop talking to the 3550's. I have upgraded to newest IOS on all, replaced gigastack cables and GBIC's, but no joy.

The 4 switches are in a stack, in a lab. There is definitely no loop I haven't seen.

Don't you love these desktop switches, so easy, so reliable..................................

 

[IPKONFIG]

What are the configurations for the ports connecting each switch?