try making it forcefully sit in the dropouts - use a tennis ball instead of cardboard? if there isn't enough surface friction, try using jagged-type washer with more friction, i forget what their called but you find similar ones on the axles of front and rear brakes. you get them integrated with most nuts.
or maybe try tightening them both at the same time? - your chain tension ain't too tight is it?
try making it forcefully sit in the dropouts - use a tennis ball instead of cardboard? if there isn't enough surface friction, try using jagged-type washer with more friction, i forget what their called but you find similar ones on the axles of front and rear brakes. you get them integrated with most nuts.
or maybe try tightening them both at the same time? - your chain tension ain't too tight is it?