The first thing to do, of course, is to check the wiring. Bare Knuckle uses different wiring color codes than Seymour Duncan, Gibons, etc. The next thing to do is to measure the DC resistance of the total humbucker and then each individual coil, it could be that one of your coils is broken or partially shorted inside (so that you aren't getting the full output of that coil).
If that all looks good, make sure again that you are splitting the "correct" coii. If you're using the bridge Crawler and splitting the outside/screw coii (the one closest to the saddles), then it's naturally going to sound more thin and quieter than the coil closest to the neck (in this case the slug coil). So you could physically mount the pickup in reverse, or you could rewire it to activate the slug coil when split, not the screw coil.
Lastly if that all works correctly, then you can use some partial coil split resistors (I suggest Googling them).
