with wood, the problem was with lignin which is tightly crosslinked, meaning that it's insoluble and organism willing to eat it has to secrete some enzymes to break it down in smaller bits that can be absorbed
depending on plastic, this first step might be easier or even happening on its own. there are already bacteria that feed on nylon but nylon starting materials are easier to digest for them
i like how on lw there's a comment saying exactly the same thing but 5x more verbose