With multiple input devices there is a good chance that your joystick is not set to device 1, which is what the weapon.xml looks for. For instance, I have a joystick, yoke, throttle quadrant and gamepad all plugged in, and my joystick is actually joystick 3. so try playing around with the numbers in this line:

<Action name="fire" control="joystick:3:button:0" />

On one of the aircraft that you have installed the weapon in, open the csweapon.xml and look for the above line.

Leave button:0 alone, and just change the numbers on the joystick. Try them in sequential order, 0,1,2,3,4 etc. until you find the one that works. You will have to manually change that line in each aircraft that you have installed the weapon in. In order to fix it for new installs, once you figure out which number to assign your joystick, then you can edit the master .dat file with notepad to assign it to the correct joystick for all new installs. But, unless you know what you are doing, I would recommend leaving the master .dat file alone, and just editing future installs. One character out of place, and all new installs will be hosed.