Similar problem here. I think I know the answer. I was leaps and bounds away from the nearest "smart" civ. So, I could not trade for them and had to learn everything on my own. Since there was no cooperation, we all were stuck.
I think, as oxymoronic as it may seem, that harder levels will be easier to learn more of the tech tree....the AI will be able to learn and I can trade (err, steal) from them and cut my research amounts in half.