Never used to have this issue on my fixie but my chain loosens a bit as soon as I start cycling. Then after 3 or 4 days it's loose enough to come off after hitting a bump.
I notice I have a washer on one side of the rear wheel between the nut and the frame and not on the other.
Do I need a washer on both is this likely to be my issue? If so can anyone point me towards what I need to buy?