WA_PubScreenName not working with PPC build

I have an odd problem with the WA_PubScreenName tag, which I am using in a call to open a window using ReAction. Here is a snippet of the code:

m_poWindowObj = (Object *) WindowObject,
WA_Title, (ULONG) a_pccTitle, WINDOW_Position, WPOS_CENTERSCREEN,
WA_PubScreenName, a_pccScreenName, WA_PubScreenFallBack, TRUE,
TAG_DONE);

This code is multiplatform and can be compiled for m68k/OS3 or PPC/OS4. If I run the m68k build on OS4 then it works and the screen is opened successfully, but if I run the PPC build on OS4 then the screen is not opened, and the window opens on Workbench.

Does anyone have any ideas?

Do you have to do something different with the os4 version like ensure the public screen exists and tick the box that says auto, or something like that?