pickup sound thin after star grounding

[brewster]

I performed star grounding technique to my american strat. Now, all pickups play correctly except when I switch to center pickup by itself - it sounds thin and muted. The wiring diagram that was shown for typical strats and for the modified is sort of like mine, except my tone 2 potentiometer is a stacked unit - 2 pots piggyback. the wiper of the lower pot goes to the switch, high side goes parallel to wiper of upper pot and fixed resistor. Resistor is attached to the pot housing. A metal film cap is also attached to housing on one end and the high side of the upper cap. The low side of the pots are unconnected as the pot body is grounded to shielding foil through the shaft nut. Anybody know what's going on?

[bam]

now that piggybacker is called a TBX tone pot.

The thin tone itself could probably caused by misconnection or fried pup.