I've googled this issue before posting.
I can't fire the KAB-500kr using my assigned joystick button. I first encountered this problem in the training mission but I solved it by changing the keyboard key assignment from space bar to the "Y" key.
Then I tried it in a mission that I made in the editor but this time when I approached the tanks with a lock on, the LA symbol flashes for a couple of seconds before disappearing. I've tried "Y" key, my trigger on the joystick (it's the assigned button on the joystick) and changing the keyboard assigned key back to Space Bar. I hold them down but the KAB-500kr doesn't launch.
I'm using the English cockpit mod v1.3 for the SU-25T.