Problem with background - studio photo

Greeting fellow Nikon lovers,I'm having problem with my studio photography. I notice that i couldn't synch flashes with a shutter speed higher than 1/200 on my d5100. I did a photoshoot this week, main lighting was a basic cameron pro 350 with a softbox center hight on model and i used another one behind model to light the background. I tried different lighting config, different aperture an iso and i always end up with one side of my background being darker (see attached image). AAny idea what may cause this. Thank you

Obviously it's Lack of light....Welcome...You haven't got a lot of image to look at...

Is it just the background or the whole of the left side that is darker..