I'm thinking three things:
- You're looking at the wrong sliders
- Your photos are off from where you need them to be in the first place
- You're shooting in JPEG, which doesn't give much freedom in post processing compared to RAW
About the first point: the "Highlights" slider controls, well, the highlights of the photo, i.e. how prominent the bright areas are (and in your case it will probably have little to no effect on the background). The "Exposure" slider controls the exposure similar to what you do in the camera (but only as far as the dynamic range captured in the photo allows).
Look at the "Blacks" slider. It controls the black clipping. It should have a range of (-100, 100) and sit at 0 by default (if it has a range of 0 to 100, you're probably set to using older process version; look at "Camera Calibration" and set "Process" to 2012). The further you move it to the left (in the negative direction), the lighter colours are clipped (i.e. set to black in the output picture); the further right you move it, the more black is "lifted" (i.e. what normally is black becomes grey). Take a look at the histogram in the Develop module. In its upper left corner there's a small triangular button. Hover over it and it will show you with blue dots on the original picture which areas are clipped to black. You can click to toggle it so the indication stays on. While it's on, drag the "Blacks" slider to see what happens with the histogram. Pro tip - you can drag areas on the histogram itself.
About the second point: I don't know what your setup looks like, but start with increasing the shutter speed - the farther away your background is, the greater effect this will have on darkening it (which however might not be very achievable with a softbox).
About the third point: if you're shooting JPEGs now, switch to RAW - RAW gives you much greater flexibility in post processing, alleviates the need to care about white balance and tolerates greater mistakes in exposure.