Andis Outliner II Trimmers

Question

The trimmers are starting to pull the hair instead of cutting it. And of course I oil them as I should and I even went to the extreme and changed the blades but still the problem exist. I really don't know how old the clipper itself is but its been well maintained. And from what I hear they usually last a while when these are the circumstances. But yet I don't understand why they're pulling the hair. Can you help?