Mistakes DO happen.
But they ARE rare!
However, the vendors receive the pen kits in bags of ten kits, usually. So, if a person in the manufacturing line was making a mistake--missing a part in the kit---they probably did it for that whole bag. So, it is not surprising if you find several "bad" kits together.
IF it is one of Exotics' kits and you call me, I ask lots of questions. This is NOT because I doubt you, I am trying to determine how much of my inventory MAY be infected with the problem you are having. It is much more economical for me to check my remaining kits than it is to have to ship product that has an error. AND as a customer you are happier receiving CORRECT, COMPLETE products.
So, I ask a lot of questions to see which kits I need to check before we ship any more!
Rockler has gone through a lot of changes in their pen kits in the last two years. So, it is no surprise to find they had some issues. Post pics of your "not quite right" kits and we may be able to help you "fill them out".
Good Luck,
Ed