[pilif]

The fix is to clean the stems with alcohol. That will get rid of the corrosion building on the contacts.

Also, as a preventative measure, before putting them in the case, make absolutely sure they are dry.

As a runner, mine tend to get sweaty and if I put them in the case with even just a hint of salty sweat on them, the current flowing through the connector will corrode it very quickly, necessitating the cleaning next time.