I use a fuji s5 pro the problem is the sb 800 nikon flash for some unknown reason the flash fires once and then wont fire when shutter is pressed again

You did not give enough information. If the flash fires when you press the shutter button then it seems to have taken a photo. You do not tell us what happens when you press the playback button on the camera or what camera you are using with the flash.

If you tried multiple batteries that you know are good and the flash will not fire while on the camera or via the test fire button, this unit will have to be returned to Nikon for service and repair.

normally is the bulb , flash tube , you need to replace or maybe is the scr transistor most popular problems for this model .

Are you saying it fires on its own random manner? Or when you press the shutter it does not fire? Please help us help you, provide much more information.

sounds like a short in one of the circuit boards, most likely one or seveal of the ribbons and boards will need to be replaced to correct the problem. Send in for repair expect to pay up to $200 for a Nikon factory overhaul

Does flash fires by itself without connected to camera?
You have pushed it all the way into the hotshoe and locked it?

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Send that back to Nikon for service. http://www.nikonusa.com/Service-And-Support/Service-And-Repair.page

I have 2 SB-800s. When batteries leaked in one Nikon took forever to fix it, so they eventually sent me a refurb. Then they sent my original back. Which was great, until recently it developed this problem.

Another possibly related side effect is the modeling flash does not burst -- it only fires a single (very bright) flash.

The capacitor argument sounds likely. Coincidentally I also had a problem where the flash would fire sporadically -- but that was due to scuz buildup on the contacts.

Sorry I don't have a real solution for you.

Definately a settings problem. The camera will only syncronize on one specific speed. Youe best bet is to use auto. For manual, you'll have to set the shutter speed at the sync speed.

Flash Sync Speed

up to 1/250 X-Sync Speed

up to 1/8000 FP High Speed Sync

Red-eye reduction
Red-eye reduction with slow sync
Rear-curtain sync
Slow sync
Front-curtain sync (normal) Flash Sync Modes

1) i-TTL: TTL flash control by 1,005-pixel RGB sensor, built-in flash, SB-800, SB-600, SB-400: i-TTL balanced fill-flash and standard i-TTL flash
2) AA (Auto Aperture-type) flash: Available with SB-800 used with CPU lens
3) Non-TTL Auto: Available with Speedlights such as SB-800, 28, 27, and 22S
4) Range-priority manual flash; available with SB-800 Flash Control

make sure your white balance is set to "flash" on both cameras