Originally Posted by
WeirdBits
We need a bit more info for an accurate diagnosis, it could be a number of things or a combination of more than one. Glue type and/or age, over sanding of the pocket and/or neck heel, stain or finish on the heel and/or pocket, finger grease or other contaminants inhibiting adhesion, clamped too tightly and squeezed out too much glue, uneven pocket or heel, temperature and humidity when glueing, movement while setting etc. More info and pics may help sorting it out and determining the cause.