If you photograph someone standing against a wall with light coming from in front, their body blocks some of that light and a shadow lands on the wall. The closer they are to it, the sharper and darker it is. That is the whole mechanism.
Why it fails the check
Every specification asks for a plain, uniform background, and uniformity is measurable: sample the background region and look at the spread of brightness. A shadow is a large, dark region on one side, and it fails on a number rather than on an opinion.
It also confuses head detection. The edge of a shadow is a strong boundary in the image, and around dark hair the real edge of the head and the shadow edge can blur together.
The fix, in order of effectiveness
- Move away from the wall. A metre is usually enough; two is better. The light spreads around you and the shadow falls off the edge of the frame or diffuses to nothing.
- Use a large, soft light. A window casts a much softer shadow than a bare bulb or a flash, because light arrives from many angles.
- Face the light squarely. Light from one side throws the shadow sideways into the frame; light from in front throws it directly behind you where your own body hides it.
- Turn off the flash. An on-camera flash is the sharpest shadow source available and it is aimed to put the shadow exactly behind the subject, visible around the edges of the head.
What does not fix it
More light from the same place. Brighter light makes a brighter shadow edge, not a softer one.
Background replacement. This is the important one. Replacing the background gives you a perfect colour behind the head - but the replacement works on the segmentation mask, and where a shadow has merged with dark hair the mask itself is wrong. You get clean colour and a chewed hair edge.
Brightening in an editor. Lifting the shadows also lifts noise and flattens the face, and the uniformity check often still fails.
Shadow on the face is a different problem
Worth separating. Background shadow is fixed by distance. Shadow on the face - under the eyes, beside the nose, under the chin - is fixed by moving the light, not the subject, and no amount of background replacement touches it.
If the eye sockets are dark, the light is above you. Face a window instead.
The check that matters
Look at the photo before you crop it: is the wall the same brightness on the left of the head as on the right, and above it as beside it? If not, you have a gradient or a shadow, and it is quicker to step back and take another frame than to fix it afterwards.
IDShot measures background uniformity separately from background colour, so a wall that is the right colour but unevenly lit is reported as what it is - and can be fixed before it becomes a rejection.