Mine never seem to produce as much gas and swell up either , perhaps were just not patient enough , I remember reading somewhere here that it can take up to four weeks in the warm phase to get some of them to produce enough gas , end even then , the texture has to be spot on so it is pliable enough to expand , but tight enough knit so the gas doesn't escape.
It seems like a lot of things have to fall into place to get the desired outcome , it doesn't seem to affect the flavor though , swelled up or not , they can still be good.
