The European Central Bank's plan to buy bonds is proving more successful at keeping borrowing costs for France and Belgium near record lows than persuading investors to lend to Spain and Italy for less.

Spain's three-year yield is back up to 3.83 percent after dipping to 3.37 percent on Sept. 7, the day after ECB President Mario Draghi detailed his proposal to buy unlimited debt for countries that agree to economic conditions in return for help. Since then, investors have lost 0.1 percent on Spanish debt repayable in three year or less, and made 0.1 percent on Belgian notes with similar maturities. The cost of insuring French debt against default has declined 24 percent, almost twice the 13 percent drop in Italian default-swap costs.

"What Draghi has done has been beneficial to some degree, but there's still skepticism in the market because Spain hasn't taken the final step and asked for help," said Adrian Owens at GAM Ltd. in London, which oversees $62 billion. "It doesn't change the fact Spain still has a huge problem to tackle. France and Belgium are seen as a safer play."