Make sure that the button isn't registered in the block list (found in settings). Otherwise, FoxVox won't pick it up when it is pressed. Some other settings in the PTT could also affect it such as the 'Isolate' option which limits PTT to not work if any additional keys are pressed besides the bound PTT Keys.
Besides this, what you describe doesn't seem to be a FoxVox issue. As you said, it picked it up when you assigned it to the PTT. Make sure the hardware/drivers are passing through the button accurately and consistently on press each time - FoxVox doesn't do anything with the input besides monitoring the button states passed in by the OS. Verify it's not being affected by any controller macro logic or third party apps.