I think your wheel nuts are not biting down enough to properly resist the tiny flexes of the bike while riding. Is the chain too long? The simple answer is that the axle bolts need to be torqued up tighter, so they grip the frame better. It looks like you have to move the rear wheel to the back-most position. Could it be that instead of these I have a striated nut? Your hub has an arm that keeps it from rotating but which can also help to protect it against moving forward.
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Is the chain too long? As pictured, they don't seem able to bolt through any holes and retain the axle. Post as a guest Name. A short length of metal tube slipped over the wrench will give you more leverage. Each time I put it in, I align it as much as I can making sure the chain is straight the cogs are aligned and then I press the axle nuts as much as I can trying to keep that alignment.