Problems could be on the horizon for the subprime mortgage market, where a rising number of borrowers appear to be having trouble paying back their loans. "The increase in delinquencies is not severe, but nevertheless bears watching," said Credit Suisse First Boston in an October update to its home-equity index. Some 14.91 percent of the subprime mortgages tracked by the investment bank were at least 60 days past due at the end of the third