Hi Somon,
This might happen for two reasons
1. If the data in both the tables are not properly arranged or sorted.
2. If the values of the data are not same to join fo eg. Table A consists of 'Fire' and Table B consists of ' FIRE'. So try checking the values of the data.
I answered guessing that the preview what you are seeing is not afterjoining rather it is before join
Let me know if my answer served your purpose.
Regards,
Karthik