Samsung SD card problem

[AC Question]

Hello
I am having in focus m370i marshmallow and bought a Samsung 32gb SD card Now I am not able to set may default storage to SD card their is no such option in my settings and whenever I download any thing after 10 or 5 min it start showing that you SD card is unavailable for download pls help
I had tried putting another SD card but after that also I am not getting setting to change storage location Please help