Hmmm. Are you binding both buttons to the same command in the PTT? If so, then both buttons would be required to be pressed together - at least how the program works currently. If you try using just one (either UHF or VHF) as a test, does it work consistently? If so I may need to add support on the PTT to allow for either AND/OR condition for multiple buttons. In its current state ALL defined PTT buttons must be engaged together at the same time - it's not an either or.
Aside from this, you could test if there's a joystick detection problem by testing with a keyboard key instead and seeing if it works consistently. You should see the PTT engage and disengage by the icon turning green whenever the button is pressed. Try testing with some simple scenarios and we'll go from there.
Let me know what you find.