Safety relays are generally pretty reliable.

Failing to switch on, is usually down to a problem with the enable/e-stop circuit. First thing I'd do, if they appear dead, is to bridge the relevant terminals, and see if they kick into life. If they still don't, then it's dead. Although voltages may measure OK on the offending circuit, some relays increase the current when switching on, so they may still be the fault, which is why bridging terminals directly is a more reliable diagnostic.

As Mud mentions, relay contacts have a minimum current rating, as they rely on arcing to clean the contacts (sometimes referred to as contact wetting). If you regularly switch below the minimum current, the contacts eventually oxidise with their resistance gradually increases until they won't switch the load.