Problem with IPac VE

[LeedsFan]

I've come across a rather annoying problem with my Defender bartop project which uses an IPac VE for the interface (USB). Basically it cannot seem to handle the thrust and fire buttons being used at the same time as the up/down on the joystick. What happens is the ship gets stuck on the screen (with up or down still held) and I need to release the stick and then move again to get it to move on screen. Obviously this is totally unplayable.

I thought it might be an issue with the game itself but it exhibits this behaviour in the Mame game selection screen also. I can move up/down the game list and press thrust constantly... no issues. I can move in the list and press fire constantly... no issues. But as soon as I simulate Defender gameplay by moving in the list and pressing both thrust AND fire the movement in the list stops. And again I need to release the joystick and then move again. 

Anyone have any ideas? I've checked the wiring and it all looks fine. All the earths are daisychained. The joystick is making contact fully on each movement with the leaf switches (it's a Williams two way reproduction).