I'm having the same issue,
Tried 4-5 different SD cards, I'd literally insert and re-insert (har har that's what she said) multiple times and the computer doesn't even recognize the blasted card, and then somehow by chance, it does recognize it and then it works.
I thought perhaps I just wasn't putting it in far enough (har har that's what she said again!), but it turns out it works on the first try every time on my friends 13 and my other friends 15. Sooooo yeah, weird. Figured it was hardware related but I'm too lazy to book an appointment with apple lol,