Just a thought - is your action super low? If it is then the strings could be getting choked when you slap. Again it really depends on the bass. A friend of mine has a mid-70s Jazz (passive) and it is a slap monster (no discernable drop in volume when switching to thumb). But if you listen to Marcus Miller on "Just The Two Of Us" you can hear a pretty big drop in presence when he switches to slapping (this was before he had a preamp installed in his bass).