A likely cause is a small broken fragment from an SD card lodged in the slot, blocking the spring-latch mechanism. In the reported case, an older damaged card would click in, and closer inspection showed a tiny piece had broken off the card and was presumably stuck inside the slot.

If multiple normal SD cards won’t latch, the problem is probably not the cards themselves but an obstruction or damage in the slot. Carefully inspect the card and slot in good light. If you suspect debris or a broken plastic piece inside, avoid forcing cards in, as that can damage the contacts or latch.

If you can’t clearly see and safely remove the obstruction, it’s best treated as a repair job. The camera may still function if the door holds the card in place, but that’s only a temporary workaround.