Paul,
Me thinks you are applying too much ca. When you put it on the blanks it also goes on the bushes and over the gap between bush and blank. We know that ca doesn't stick too well to metal and generally the strongest bond in this scenario is that of the ca on the blank. Normally the ca will snap just proud of the blank leaving a ten second job of removing the surplus by rubbing it on a piece of scratch. If the build up of ca is greater then the glue delaminates off the blank and sticks out like a foreskin! If the build up is like this then it can crack off the blank as well.
That's why I think you are putting too much glue on the blanks. Now this bit isn't contentious at all and I know it won't provoke any response, but I think that a lot of people put far too much ca on their pens. 
