How are you triggering the slave flash? Are you using the meter on the flash?

The right way is to set the built-in flash on the D80 to manual setting and low power (1/16 for example). You will also have to set the camera to manual and work out the right shutter speed and aperture and ISO for yourself.

The wrong way is to use the camera's built-in flash on auto. The camera uses a pre-flash to evaluate the exposure before setting off a second, precisely dosed flash with the shutter open this time. The slave flash thinks the pre-flash is the real thing and fires while the shutter is closed.

I suspect that you are using the flash meter on the slave flash. With the head set direct, the pre-flash triggers the slave, the meter cuts the flash off quickly and the flashgun has enough energy left to fire a second time when the real flash comes along. With the head set to bounce, much more light is wasted and the slave flash has no energy left to fire a second time.

Yes you do need to buy a fully compatible flash.