Missing data in Cassandra database

Question

I have deployed Cassandra server from google cloud marketplace with the below configurations Instance _count:7, machine_type:e2-highmem-4, zone: us-east4-a

The configuration details and queries for creation of the keyspace and table are as follows:

CREATE KEYSPACE migration
WITH replication = {'class':'SimpleStrategy', 'replication_factor' : 3};

CREATE TABLE migration.mock_data ( 
  id uuid PRIMARY KEY, 
  first_name text,  
  last_name text, 
  email text, 
  gender text
);

COPY migration.mock_data (id, first_name, last_name, email, gender) 
FROM 'MOCK_DATA.csv'
WITH HEADER = TRUE;

And tried loading some test data of 1000 rows for my migration project, but surprisingly I found some data missing from the database.

Output of the select query is given in this image

Any inputs on this issue will be helpful. Thanks in advance.