In relays which have two switches in series: If one of the switches loses its vacuum, it will have a low breakdown voltage. Two switches in series is used to achieve the additive effect of two 10,000 volt breakdowns adding to give over 20kV. So what has probably gone wrong is one of the switches has lost its vacuum, perhaps due to a small crack or a bad seal. Try to remove some of the epoxy on the end were the reeds are soldered together and then test them individually to see which one may be bad.
