The NI PCIe-6321 X-Series board has 4x general counters, which can be used for PWM generation. However, when conducting tests using the attached VI we were unable to general PWM signal using all the counters. It would generate when Dev1/ctr1 and Dev1/ctrl3 were selected. Is there any DAQ configuration required or something?
I would recommend looking at the examples that come with LabVIEW. There are multiple examples of how to generate pulses under Help > Find Examples. The Counter Output examples can be found under Hardware Input and Output > DAQmx > Counter Output. Looking at these examples will show you the correct way of formatting the code, and you can run these to make sure that there are no issues with your hardware.