I have come across this annoying quirk that has taken me days to track down! I would call it a bug; if you could point me to a document that explains otherwise, please let me know.
If I attempt to use a property node to set values for "White Reference (volts)", then the new value is NOT used by an "IMAQ Snap" call that immediately follows; it seems to get applied by the hardware (PCI1408) only _AFTER_ this call.
If, instead, I replace this property node with the old "IMAQ Get/Set Attribute" which is no longer available in the 6.1 Functions Palette (actually I grabbed it from inside 'IMAQ Snap'), then the property is correctly changed for all subsequent snaps.
If you want to look into this, but are unable to
replicate, I can email a program (just not handy at the moment).