I discovered the problem with Coopers personally, way before widespread use of the internet. The problem with many is weak springs, failure to feed the last 7-8 rounds.
Even with the spring problem, there were no other visible problems with the mags. Putting a spacer under the spring to increase tension would make it feed all rounds, though limiting the mag to 20 rounds. This confirmed to me the problem was springs. The bodies looked perfect.
Way back in the pre-ban days you could not find replacement springs or other parts. Mags were simply discarded and replaced.
Another reported Cooper problem, one which I have never seen, was some had only 3 spot welds on each seam, rather than the required 6. These are to be avoided. But if the welds are correct, drop in a new spring and test them.
On the other hand, I have a lot of D.L.Sanchez mags (more than 20, possibly 30 or so), and NO problems with them, even with the original black followers.
If it makes you feel good, put new springs and green followers in them.