Some U.S airlines rolled back many of their broad fare increases earlier this week, which is a sign that demand is softening.Continental Airlines floated into the market a fare increase last Friday by raising prices $5 each way on all of its U.S. flights. The move was matched by most major airlines, but not by budget carriers like Southwest Airlines, which doomed the effort. The unsuccessful attempt at a fare increase is the latest setback for the whole industry, which has been contending with jet fuel prices that have risen by 30 percent since January.With traveler demand showing signs of a slowdown and competition as intense as ever, the U.S. carriers appear to have little power to pass the higher costs on to consumers. "We continue to doubt the effectiveness of subsequent domestic fare increases," said JP Morgan analyst Jamie Baker in a research note. He said despite 20 fare increases since January 2006, domestic flights cost only about 1 percent to 2 percent more than a year ago.