This may be a case of overtraining (this is a term from neural networks). To explain this, I'll use an example from a cartoon I saw once.
There was this intelligent but lazy shark that didn't want to go out hunting. So it bought a robot to seek out cans of fish. It showed the robot a picture of a can of fish and the robot went out to find a can of fish. Unfortunately it came back with an empty can of fish, obviously this was not what the shark wanted.
There was a dial on the robot to adjust the robot's intelligence; the shark set it to the highest setting, cleared the memory, showed the robot the picture of the can of fish and sent it on its way. It returned with a picture of a can of fish.
See, the robot was overtrained, it could recognize a picture of a can of fish but not can of fish in the wild. In the same way, doing easy homework problems until they seem easy may just mean you can do easy homework problems. You need to understand the problems or just knowing how to solve them could leave you high and dry at exam time.