That is “messed up”, as they say. The only reason I can think of that might result in that would be misadjusted trimpots. The Bassballs has two filters, that sweep in the same direction. Trimpots on the board set how far up the filters sweep. If they are set for the filters to be TOO sensitive, then anything more than soft picking will have them sweeping up where there isn’t very much frequency content at all, which will sound faint.

You might want to try playing around with the trimpot settings – with the sensitivity up full – to see if you can dial in an acceptable sweep with your typical picking style.