The reason this happens is because as you practice the same thing over and over again you start to focus your mind on why you cannot play this chord or note correctly. You end up trying hard to position your fingers in a certian way to reach a chord or note that you probably already know well. If you can try this experiment what i am trying to explain might become more clear. Walk up and down the stairs without thinking what you are doing, then walk up and down the stairs thinking about every move you make. You will find that you movements in the first instance are smooth and in the second instance sticarto, Once you have trained your body(fingers) it knows what to do, so two hous of stairway to heaven one night pays off the next day when you let you fingers do the talking.
Spooky but it works.
Garp
The Inner Game for musicians