My flash only works when I'm shooting in auto mode. Why is this?

I have a Nikon D5100 & Bower SFD926N Digital Shoe Mount Flash. I'm trying to teach myself photography & get out the auto mode of shooting. However the minute I switch my camera to Manual mode or Aperture, shutter. My flash stops working. Does this mean I need a Nikon flash instead? And if so I'm looking at either the sb 600 or 700, but the 600 seems more in my price range.

I do all types of pictures with my camera from indoor with a backdrop, outdoor portraits & events with low light.

Appreciate any advice, thanks!

When you are manual mode you have calculated the exposure yourself without the flash or you are using external floodlight.

When you are shooting with flash, you need to assure that the shutter speed is at the cameras flash sync or slower.

That usually means that you shoot in the manual mode with the shutter speed set to 1/200th second or slower. You can also use the shutter speed priority mode with the shutter set to 1/200th second

All this is in your user manual.

I would suggest you are wasting a lot of time with your flash.

Better you learn how to balance the cameras ISO, shutter speed and lens aperture to get the best exposures without a flash here.

http://camerasim.com/camera-simulator/

And start honing your skills with your image composition.

http://photoinf.com/General/Robert_Berdan/Composition_and_the_Elements_of_Visual_Design.htm

http://www.photographymad.com/pages/view/10-top-photography-composition-rules

A flash unit is used so rarely (unless you are shooting events every week), it is something that should be a lower priority when it comes to "learning photography"

Have you tried reading the manuals for the camera and the flash?

amazing what you can learn from manuals

You have to find the sync speed of your flash. You can't have a fast shutter speed with flash. It's usually 1/200 or 1/160.