>>>Make sure that all of the masters are inherited from mmMasterPage and not from the standard master page. You would need to add some Mm using statements but they need to all be subclassed from mmMasterPage the same.
>>
>>The IASLeftNav was just a MasterPage, not mmMasterPage, but that still hasn't worked and I've tested it by setting a breakpoint in all three page_loads. The debugger then jumps straight from the content page's page_load to the OuterMaster, skipping over tha IASLeftNav completely.
>
>All three page loads run on mine and in the expected order. Default, then nested master, then the outer master.
>
>I think you may have AutoEventWireup set to false. Check in the aspx source of your nested master page at the top where the page directive is and the master page is indicated. See if you have a directive - AutoEventWireup=false. Change that to true.
>Tim

It was set to false, but changing it to true hasn't made any difference. :(