relay contact fails (suspect sparking)
From your description you have no diode across your solenoid coil.
That will cause a very large voltage to develop across the contacts of your relay as they open. This cause a spark and will degrade your solenoid contacts.

Simply place a diode across the solenoid to protect the contacts:







simulate this circuit &ndash; Schematic created using CircuitLab

Almost any general purpose diode such as a 1N400x series would work as it only has to tolerate the pulse power as the magnetic field decays.