Hi Eric,

The width of the tang is the problem. Whatever the original width of the tang (ignoring the barbs), a slightly wider tang is used on each successive refreting job. The operative word here is 'slightly'. One or two thousandths of an inch is the tolerance level. Too narrow a tang and the neck will develop a dished profile under string tension. Too wide a tang and the neck will have a back bend.

Often, a good luthier can correct a dished neck by judiciously varying tang sizes during a refret. Any competent luthier should have little problem with finding the correct size.

Good luck,
Ray