I recently migrated from my C5 to a C7 and have had all sorts of Z-wave issues initially, but once I removed ghosts, I have been Ok. Last weekend I installed a lock and it seemed to pair only partially and the hub was very flaky, but a removal of the device and a full reset of it helped to pair it again with S2 security which helped.
Is your lock paired with S2? Also, I discovered an app I want to try myself to keep it connected [Release] Reliable Locks
