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?