I've got dozens of S&W factory magazines for 9mm, 40 S&W and 45 ACP. Some were bought new but most were used when I got them. The springs in all were serviceable. The biggest issue I've found with S&W magazines is the followers. It seems it takes S&W about three tries to get a particular follower design right.

For example, with the 40 S&W I've got magazines with black followers, yellow followers and blue followers. The blue followers are the most recent design and offer the best reliability. The same is true with the single stack 45 ACP magazines.

If you are having magazine problems I suggest, in addition to changing springs (never a bad idea, btw) you should also make sure you have the most current follower design. One call to S&W or Brownells will get you what you need

BTW, Mec-Gar magazines are excellent! They are the only after-market magazines I trust without question. They make the OEM magazines for a good number of gun makers (to include Beretta) and their products are well designed, well made and they sure are cheaper than S&W factory magazines.