Setting up a new shard cluster consisting of 6 shards, each a replicaset. Five of the 6 shards were added OK by mongos, but the last one would not work. The mongos logfile said this;
<quote>
Wed Dec 7 12:42:18 [conn23] addshard request

{ addshard: "replset5/mon-8088.recfut.net:27019,mon-8089.recfut.net:27019" }

failed: couldn't
connect to new shard mongos connectionpool: connect failed replset5/mon-8088.recfut.net:27019,mon-8089.recfut.net:27019 : connect failed to
set replset5/<node7>:27019,<node8>:27019
</quote>
There was nothing in the mongod logfiles on node7/node8 nor in the mongoc log. The replicaset could be connected to from the commandline:
<quote>
$ mongo --host replset5/<node7>:27019,<node8>:27019

The fix was to restart mongos.

The replicaset had probably been attemped to be added earlier, and failed because the replicaset was not yet live.