Just like how ants are viewed as one of the more intelligent insects, their tiny brain cells can only learn so much. The level of intelligence for the human brain is also limited due to it's size and thus, it is very difficult for us humans to become more intelligent than we are as a human race assuming that the average size of our skull remains the same.
Einstein's brain was smaller than average but he was also more intelligent than average. This shows that there is definitely room for our brains to become more intelligent through more efficient wiring, and through a reward system that fixates moreso on the pleasure of learning than on pleasures from activities which don't enhance one's understanding of anything. Both factors would make a person more intelligent.
And beyond that we will soon have technology to store memories on harddrives that connect with our brains, allowing our intelligence to be enhanced even further. And also preventing (or since the physiology would still be there I suppose you could say getting around) diseases like Alzheimer's since the memories would be safely stored on a hardddrive.