Cameras are built to assume that all scenes are an average of 18% gray. Most of the time they are close, but if you are using a black background or an all white scene, like a snow scene, they come up wrong in exposure, or assessing that situation. They fall back to assuming that it should be 18% gray. Now days the cameras take many more things into consideration, and assess the situation in many ways in order to alleviate that but still have serious issues.
If you use some sort of program that you can alter your photos, whether it be Gimp, or Photoshop or Elements, all you really have to do is darken it slightly or move the "Exposure Slider" slightly. If you are using your cell phone, they all have built in software to do the same. Its a simple fix.
Another fix really easy fix is to use a medium to Light Gray construction paper inside your tent as a background, it will make your camera happier.
For future work, your set up should be consistant everytime you set it up, if you have the abilty to control your exposure in a manual mode, play with the settings. Set the settings that are variable (fstop....Shutter Speed...and ISO) via what it tells you it is going use in an auto mode and then turn down the fstop for white (to a larger number) and turn it up ( to a smaller number)
for black. And just maybe one or two stop at most. Write this all down and put it in your tent, given you use the same lights at the same distance, it should be the same all the time.
Also, if you ever need to youtube anything, you will not find your answers for using your "lightbox" under that name. Technically a "lightbox" is for rear lit box/table for viewing slides. What you have is a "Light Tent"....and not that what you call it makes any difference but if you ever need to reference any outside materials, it might.
You will find a lot of experts in this area, some have great advice, others not so much....try them all and use the simplest solution that gets you where you are happy.
Just my .02