On my warwick thumb, I can't get my action somewhat respectably low without these strings buzzing up until about the 6th fret. Same thing happens on frets 18-23 or so. I've tried straightening the neck via truss rod a few times, but to no avail. It's an older thumb with a wenge neck, five string. I've tried the bridge, nut, and truss rod, and still it buzzes.