Got one today for my son and have the same problem.
If i insert an sd card the tab doesn't see it.
I have to wiggle it a little and then it's mounted but when i release the pressure of the card it says that the card has been removed.
I solved it by applying 4 layers of officetape to the printed side of the mini sd card and pushing the card gently in the socket.
seems like the bracket of this tab is to big for the card and because of that the the pins can't reach the sd card copper pins on the sd card.
By applying the tape i actually made the sd card thicker so it's pressing more towards the bracket pins.
Hope this helps.