Back wheel continuously coming loose I’m a total amateur when it comes to biking... I recently purchased a new Bergamont Summerville N7 and took it out today for the first time longer than an hour (I’ve had it a week and been to the shops twice with no issues)

However today about 40 mins in the back wheel became incredibly stiff. When I looked I noticed the wheel had shifted position and the tyre was rubbing against the kickstand. 

After further investigation it was obvious that the left ‘bolt’ attaching the wheel to the frame was loosening no matter how tightly I fixed it. One of the ‘washers’ is different to the other (which is the one causing an issue) 

The one causing an issue also seems to be shearing into the frame itself... I have pics but for some reason this won’t let me upload the images.

Any help would be greatly appreciated before I email the store I purchased from.

Thanks in advance
I suspect that the hub bearings are wonky and are causing the axle to twist.  This will tend to unscrew the right side fixing bolt.