Here are (4) main reasons your child’s DNA may not match your DNA at every marker:

(1) The match is from the other biological parent.
(2) The match is related to both you and the parent of your child, but doesn’t share enough DNA with you to show on their genetic makeup.
(3) The match shares a small identical segment, along with an identical by descent segment that they share with the child.
(4) The match shares a small segment with your child that is coincidentally identical.