Try couple things:
1. Try (may be borrow from a friend if you don't have one lying around) a different brand name card. Some generic brand cards (especially from eBay) are little flimsy and does not provide proper contacts. It happened to me on both HTC M8 and LG G3. Tried SanDisk card and all went well.
2. Try formatting the card using PC into Fat 32 format (you can always reverse this formatting to ex-fat) and see if this make any difference.
3. This one sounds very basic but under storage, sometime you have to manually click on "mount SD card".