"Resting" magazine springs after being compressed for a very long period of time will not return them to the same contition they were when they were new. If the springs would not feed rounds correctly after a period of time, there is little that you can do to return those springs to "life". Pitch 'em.
As an aside: I was asked to check several firearms in an estate and found a flint lock rifle that was cocked. As far as anyone could remember the rifle had to have been cocked since sometime in the late '30's when the oldest son had been playing with it and forgot to lower the cock. The rifle was built in the early 1800's. I lowered the cock, thinking there would be no spring tension left and was suprised to find that the lock would still throw a good shower of sparks. (I tried the lock with the permission of the owners and only after making sure the rifle did not have a charge in the barrel.)