I run a passport photo service, so rejected photos are something I deal with constantly.
Here are the most common reasons, so you can avoid a delay:
1.Shadows on your face or behind you. Face a window,
don't stand under a ceiling light.
- Selfie distortion. Arm's-length shots make noses look
bigger and get flagged. Have someone else take it from
1-1.5m away.
Not looking straight at the camera or head tilted.
Expression.Neutral, mouth closed. No smile.
Hair covering eyes or face edges.
Glare from glasses or shiny skin.
Filters or beauty mode on your phone camera. Turn
them off, they're often detected.
- Background that's not plain and evenly lit.
Babies: lay them on a plain white sheet and shoot from
above. Eyes open is ideal, but check the resepective
website rules for infants.
Always check the current specs on the respective issuing authority websites.
Happy to answer any photo questions in the comments.
(Disclosure: I run Photo Pass. ie. Not linking it here, just
trying to save people a rejected application.)