Hi Joseph,
It sounds like you have things set up correctly, but just to double check:

1. You are using the simulink block called PCI-6229 National Instr. Analog Output
2. When connected to AO-0 your settings in the block are:
3. Channel Vector=[1], Reset vector = [1], Initial value vector=[0]

If those settings are all correct then we can go on to a few more possibilities.
How long are you trying to change the analog output value for? If the analog output block is set to run at 1Khz and you are passing it input from stateflow which runs at 2KHz and only changing for 1 frame then it’s likely that the analog output block is never seeing your change. If you try to change the output for more time then will it always work?
Have you tried this using the digital outputs?
If none of that help please send an email to support@bkintechnologies with a copy of your model.
Cheers!
Duncan