I was having the same issue. The card reader worked when I first got it, and then didn't respond at all to a card. You can run a hardware diagnostic by powering down the computer, and then holding the D key when you power it back up. Click the T key to run the test. This can help rule out whether the hardware is having an issue or not.
It turned out my hardware passed the test, so I looked around on the Internet. Someone else said they had issues with SanDisk SD cards and the micro SD to SD card adapters. The card or adapter will keep one of the pins stuck after removing it. As suggested, I carefully took a toothpick and pressed on the far left pin in the SD card slot. It loosened it up enough, and now my card reader works normally again.
I'm not sure if it will work for you, but I would give it a try.