Sometimes, especially in more advanced levels, learning isn't very linear and there can be periods when it appears that progress isn't being made. These plateaus, the fossilisation you mention, don't necessarily mean that nothing is being learned. At lower levels it is easy to see progress- you don't know the past tense, then you learn it. Higher up, it becomes a lot of small points and fine tuning. Try comparing what you can do now with what you could do six months ago.