Nissin Speedlite Di466 won't fire?

I have a Nissan Speedlite Di466 flash gun. I use it on my Nikon D40. However, I'm having a problem where it's pot luck whether it will fire or not and when it does fire, the images are extremely underexposed and only 1 in 20 or so will be exposed properly when it does manage to fire. My camera is in TTL mode and the slave isn't turned on. I don't know what's the matter. I've had it for a while and as far as I remember it has always been like this. It's out of warranty though because I got it Christmas 2010.
I've also made sure that the flashgun has new batteries in it.
I usually use my camera in either Automatic or Program (mostly program though). I've read that sometimes flash guns won't work above or below a certain shutter speed but I'm not sure what that is for the flashgun I have. I've googled but to no avail.

Does anyone know what the problem might be?

Is the Di466 a TTL flash? I just checked and it is.

You must shoot with your camera in either the manual or shutter priority mode to assure that the shutter speed does not exceed the flash sync of the camera (the D40 is one of the very few that can sync at shutter speeds faster than 1/250th second