The more sand you take out the more roll you get so conversely the less sand the more spin.
Ever notice how when you nearly thin one across the green its spinning like a mad yoke, thats coz you got the ball first.
Its tough and it takes practice and lots of comittment but its a nice shot to have for the slightly longer bunker shot.
Though its not a percentage shot by any means...

Are you swinging any faster/slower out of the two types of sand?