George Cayley, aka "The Father of Aerodynamics" essentially, invented the field of heavier-than-air aerodynamics and built a number of model and man-carrying gliders, all in the first half of the 19th century. He defined the four forces acting on a plane, understood aerofoils and invented a device to test them. The layout of a modern plane can be traced back to him. He even invented the "tension wheel" to keep undercarriage weight down - an invention still found on virtually all bicycles today.
However he never built a true plane. Two problems alluded him: stability/control, and power. Both problems were only truly cracked by the Wright Brothers, and it could be argued that solving these problems were essentially the Brothers' main achievement.
Cayley had no hope of cracking the power problem. Steam engines were far too heavy and internal combustion engines were decades away (even the Wright Brothers had to build their own engine).
However the problem of stability and control (they are two sides of the same coin) were definitely within his grasp. If you look at the tail on his man-carrying machine, he appears to be thinking along the right lines - his tail is more like that of most aircraft, rather than the split tail used by the Wright Brothers (which was probably the more practical considering the technology available).
So how did he manage to miss this important step? He built a lot of models over an extended period of time (almost 50 years by some estimates) - how come he didn't come up with the idea of warping the wings or adding control surfaces? I think everyone who has built paper aeroplanes has accidentally warped their wings! Isn't Cayley likely to have made essentially the same mistake?