SubStacks and message path

[Keith Clarke]

Hi folks,
I'm using a substack 'Credentials' as a dialogue box to capture login credentials for a web service (tUsername, tPassword). 

A second 'SOAP' substack acts as a library managing the various POST messages for the web service, including a function login pUsername pPassword

If I attempt to use 'do login(tUsername, tPassword)' in a button on the Credential's substack, the script throws errors - so I'm assuming that the SOAP substack is not in the Credentials substack's message path. 

So, I'm trying the call function for the first time, but get errors with call login(tUsername, tPassword) of stack "SOAP"

Any clues as to what I'm dong wrong?
Best,
Keith..