Nikon D3200 not firing SB-400 external flash?

I have recently bought a SB-400 for my Nikon D3200. I have tried different settings in which the internal flash will trigger but in the same setting the external flash doesn't trigger.

What could be wrong with the setting?

Could be any number of problems: 1) flash unit not seated properly in hot shoe on camera, 2) settings not correct on flash unit, 3) flash not charged (no batteries), 4) flash is not enabled in your camera's menu.

What mode you using? M mode? If so, your shutter speed can't go above 1/200s That is your camera sync speed. Anything above that, your flash won't fire.

Are you trying to fire the SB400 OFF camera? If so it won't work - the sb400 doesn't have an optical slave mode.

If you want to fire it off camera, you'll need a Nikon iTTL compatible flash cable.

I don't think radio triggers work with it either. Tbh it's a poor choice of flash for a lot of reasons - no bounce/tilt head, no compatibiltiy with Nikon's wireless flash system.

About the only thing it's good for is using on the camera's hotshoe.

You're better off using a cheaper manual flash & radio triggers at lot less cost.

Set flash to manual or auto mode at lowest ISO. Set camera to manual mode, 1/125s and to the flash ISO. See back of flash for aperture size or start testing at f/16. If too bright, raise f/number or step back. If too dark, lower f/number or step forward.