Engines are hard
Rocket engines are nontrivial. Not only do you have white hot pieces of metal trying to contain a high pressure burning fuel, but you also have to ensure that the oxidizer doesn't impact the walls, else it will cause a burn-through. And, that has to be done at any throttle setting. Oh, and you have to make sure that the thing doesn't begin to oscillate in any matter (Think of a whistle.).
Starting a rocket engine is nontrivial, too, since you really need to inject the fuel and oxidizer such that there isn't an explosive mixture present at any time before ignition, else the pressure wave will shatter the thing. Restarting one, especially in space, is even trickier, given that the fuel (and oxidizer) isn't always positioned correctly in the tanks (Consider ullage rockets.). And, restarting multiple rockets engines in space is even trickier, since, if one ignites first, the thrust imbalance may cause a spin or destructive torque on the vehicle.
Thus, I'm amazed that he's had the successes that he has had, and has even come close.
As a friend of mine, who wrote most of the FCS (Flight Control Software) for the Space Shuttles, once said "It is rocket science!".
P.S. My friend also compared the flight characteristics of the Space Shuttles to a number of uncomplimentary items, such as "A Buick with the doors open", "A garbage truck", or, most commonly, "A flying brick". ;-)