Hi Kumar Anish,
This problem may occur due to many reasons.
1 ) Please ensure that in both the client machines the path of the connection manager is correctly set to the location of the database in the server
machine.
2 ) Go to sever machine. In that take control panel-> windows firewall-> allow a program or feature-> Allow firebirdserver.exe in the list and press ok.
I hope that doing the above steps will solve the problem of yours.
Regards,
Justin