Can you measure the chain and check the wear? 12 links should be fairly close to 12 inches. When I got my Raleigh, the chain on that was at just under 12 1/4 inches for 12 links, and that still ran fine on the chainring, so that must be one seriously worn chain if that's what's going on. Pitch mismatch would be the only other reason I can think of, because the chain must be wide enough if it seats initially.