A replSet has three members, aa/bb/cc. In this situation , aa->bb->cc , aa is primary and the sync source of bb , bb is the sync source of cc . aa advances the lastCommittedOpTime , and it signal OplogWaiters , so bb can advance his lastCommittedOpTime , but sometimes cc couldn't advance its lastCommittedOpTime.
 
The key problem here is the _localOplogCollection always is NULL for secondary.

@oplog.cpp 
void signalOplogWaiters() { 
if (_localOplogCollection) { 
_localOplogCollection->notifyCappedWaitersIfNeeded(); 
} 
}

the only way to set _localOplogCollection is in function getLocalOplogCollection，and called here.

void logOp(OperationContext* txn, 
const char* opstr, 
const char* ns, 
const BSONObj& obj, 
const BSONObj* o2, 
bool fromMigrate) 
  
void logOps(OperationContext* txn, 
const char* opstr, 
const NamespaceString& nss, 
std::vector<BSONObj>::const_iterator begin, 
std::vector<BSONObj>::const_iterator end, 
bool fromMigrate)

but for secondary ，it returned early because of oplog Disabled. 

So because _localOplogCollection = NULL , when secondary update _lastCommittedOpTime , it cann't signal OplogWaiters.