I have not had any problem with newer pencils not blending. I didn't see any difference in the pencils except maybe a very slight difference in diameter.
I agree FWIW. I have some older sets..the ones made in US..and a couple of newer open stock pencils that I have had to replace. I haven't had any trouble blending the new ones either. I did notice a small difference in size as Dini said above..but not enough to really bring attention to. Sanford does stand behind their product..I had an issue with a couple of them breaking every time I sharpened them. I thought at first it was my sharpener( electric.) The breakage was so bad I was taking my sharpener apart several times a day to get out the broken pieces of "lead". I started using a hand sharpener..Prismacolors own sharpener at that. Didn't help. I contacted customer service and they replaced ALL the pencils I had ordered and sent a new hand held sharpener. I didn't even have to send them the old ones either. The new ones were mailed in thick bubble wrap..and I never had an issue with them at all. Prismas are my favortie..I do love my Polychromes too...but I adore my Prismas..LOL