The runup to D-day saw a massive effort to glue up plywood into gliders, and the early batches didn't hold very well. The trouble was traced to the platen presses (that held the plies together to make loose bits into solid plywood).
(from _The New Science of Strong Materials_, J. E. Gordon) "... a proportion of aircraft plywood was ungluable. The joints in such plywood, made with all due care, looked like any other joints but had no strength..."
The effect is sometimes called case-hardening, and is due to microscopic damage that prevents the glue from penetrating into the wood to make contact with the subsurface (strong, intact) wood fibers.
Sandpaper is a good treatment for the problem.