Check inside the garage board. I had a fault exactly the same as this last week. RCD would hold but wouldn't work on the test button. Tripped on Zs test.Wouldn,t trip on any RCD tests. Disconnected everything from RCD and it worked perfect. Tripped on test button and all trip times within limits. As soon as busbar and outgoing neutral reconnected fault re appeared. Turned out to be an earth nicked on neutral bar. Once that was sorted all was perfect. Not saying this is definitely your problem but worth a look.