This must sound like a very long winded subject header .. and it is
I never understood how Flash lights work .. I almost always ended up with a very over or under-exposed shot .. and after a while I started firing several test shots before I started using the flash to work out the best combination of the above without really understanding how it worked ..
It wasn’t till I read a book : Understanding Flash Photography: How to Shoot Great Photographs Using Electronic Flash by Bryan Peterson (Aug 30, 2011) .. that I realised that shutter speed makes very little difference to shots with flash .. it is more about how far the subject (double the distance you get 1/4 the light or the inverse square law), what ISO and most importantly what aperture .. while shutter speed only alters how the ambient light is captured .. there are loads of posts on the web about flash photography in full detail ..
So next time you use a flash .. decide whether you want to capture the ambient light or not by setting your shutter and ISO .. then either use the flash with TTL metering i.e. set to automatic or set things manually .. vary the FEV or flash exposure value compensation, flash intensity, aperture and most importantly stand at the right distance .. and try first and second curtain sync for special effects ..
I still never get it 100% .. but using a bouncing flash and/or off camera flash and maybe a second and even a third flash or more can all help get your shot right .. there is also a place for playing with on or off-flash reflectors/diffusers and coloured gels .. and now there are LED continuous flash units that are weaker but easier to use as they become part of the ambient light and makes the camera work much easier .. and also ring flash units for macro photography ..
Hope this helped to start you off on a search and a new project .. get the flash photography right