WD My Passport Ultra not mounting. :(

Hi, everyone! Does anyone here can help me with my problem. My new passport is not mounting on my Macbook Air. I reformatted it into Mac OS Extended (Journaled) for compatibility. It works fine at first. Then when I used it for third or fourth time, it suddeny became unmountable. It shows in the Disk Utility but it wont let me mount it. I dont know what to do already. :( 

Hi, welcome to the community.

Have you tried connecting the drive with a different cable? Have you tried connecting the drive to a different computer? Have you run a first aid on the drive? Did something happened to the drive?