got ya… yes i have not actually done it myself but PJ did… is the thru hole you are using plated?, a via? Top plane to bottom plane?
maybe it has something to do with the spacing between the XIAO and the board. Did you solder the other pins before or after?
did you place flux on the top of the sandwitch? I am assuming the solder was connecting firmly to the XIAO but was not making good contact to the board? you are going to need enough copper on the board to mate… maybe PJ can give more info
Thanks for letting us know its not working… so we can know and do better!