The only thing I can think of is that the coloured core of the pencil hardens over time or in certain conditions as I cannot replicate the issues you are suffering.
Disclaimer: I don’t have the book and I don’t have any of the pencils.
From what I can see in the pages of the AK book published here and what I’ve gleaned off the various internet forums and webpages, I am getting, have gotten and will continue to get the same type results using a decent set of colored pencils and some watercolor pencils purchased at the local art store.
I have no idea what the AK pencils cost, I imagine they are pretty expensive. Quality colored pencils from the art store are not ‘inexpensive’ but individually or purchased on sale or with a coupon I’d venture to guess they are way less expensive than those stamped with the AK logo.
This post is certainly not a dig at AK or their products, I just think that most “scale modelling” products can be sourced elsewhere at less expense. If you look around you can find either the same products (under different names) or something very similar products that produce the same end result.
AK, MIG etc. make it very easy for the modeler by providing a ‘one stop shop’. If you’re willing to pay for that convenience then those scale model specialty companies fill the need. If you are on a limited budget or prefer the ‘old school’ approach to modeling, you can find the same materials that work just as well elsewhere.
Cheers,
C.