Couple of things come to mind. The coil reject armature can sometimes slip out of its' housing. You can tell visually, if the plate is pulling in or not on an angle - not flush with the coil. This will cause issues more relating to a "chatter" - but worth checking. The spring on the relay - has it been changed recently ? If it is too strong, the coil will be fighting the armature.
If not those simple checks - look at the Solenoid Driver board connectors and housings. Make sure there are no pins loose. Reseat and see how you go.
If that still does not fix the issue - remove the solenoid driver board, and look for dry joints. If you have a soldering station close by, resolder all the header pins and also the relay.
