What is possible to happen in this case is a wear of the SD card (and/or inside the GeoExplorer).
The SD Card socket has sliding contacts and the contact patch is fixed, so as you push the card into the socket the contacts slide over the SD card contacts. Pushing into the connector to the lock position results in the pushing the card further into the socket, it then springs back to the lock position. So pushing the card further into the socket will use a different contact patch on the card than it will see in the lock position.
The sliding nature of these connectors does cause wear over time of both parts but specifically on the SD card. With a worn SD card the electrical contact on the card is not available in the locked position.
If the SD card shows signs of wear it's time to replace it. We recommend using quality cards. Not only for data integrity in general but also the mechanical quality is higher.
If the card is new and of good specification and condition, we may likely just have a defect with the socket on that particular product. To troubleshoot you can use another or a new card and check if that card connects.
If the trouble shooting indicates the GeoExplorer socket has a problem return the product for service to inspect.
Image of a worn SD card: