No output when use pymongo watch()

I try to access real-time data changes with pymongo.mongo_client.MongoClient.watch(), code example (Python 3.5.3, pymongo==3.7.2):

from pymongo import MongoClient  
DB_CON_STR = "mongodb://10.10.10.1:27017/?replicaset=rslogs"  
mongo_rs = MongoClient(MDB_CON_STR)  
MDB_SCHEMA = 'logs'  
MDB_COLLECTION = 'rqs'  
for change in mongo_rs[MDB_SCHEMA][MDB_COLLECTION].watch():  
    print(change)   

and don’t get any output.

Replica Set consist from five mongod instances,
Earlier two weeks ago used db.version() 3.4 with set featureCompatibilityVersion: 3.2,
then updated to db.version() 3.6.9, WiredTiger storage engine, pv1, featureCompatibilityVersion: 3.6, logSizeMB: 51200.

logs contain:

2019-02-25T15:02:55.353+0000 I COMMAND [conn30246] command logs.rqs command: getMore { getMore: 6649875775522786240, collection: “rqs”, lsid: { id: UUID(““) }, $clusterTime: { signature: { hash: BinData(0, 0000000000000000000000000000000000000000), keyId: 0 }, clusterTime:Timestamp(1551106974, 13) }, $db: “logs”, $readPreference: { mode: “primary” } } originatingCommand: { aggregate: “rqs”, pipeline: [ { $changeStream: { fullDocument: “default” } } ], cursor: {}, lsid: { id: UUID(““) }, $clusterTime: { signature: { hash: BinData(0, 0000000000000000000000000000000000000000), keyId: 0 }, clusterTime: Timestamp(1551106971, 9) }, $db: “logs”, $readPreference: { mode: “primary” } } planSummary: COLLSCAN cursorid:6649875775522786240 keysExamined:0 docsExamined:16 numYields:16 nreturned:0 reslen:210 locks:{ Global: { acquireCount: { r: 36 } }, Database: { acquireCount: { r: 18 } }, oplog: { acquireCount: { r: 17 } } } protocol:op_msg 927m

In what could be the problem?