hi Tarun,
Just to check have you implemented any password policy in the ECC system as some of the occasions the change in password is not permitted again.
do refer below help.sap.com links may be there might policy enforced like Allow Users to Change Their Own Passwords & Allow Old Password as Part of New Password might impact the change of password.
Configuring the Security Policy for User ID and Passwords - Identity Management - SAP Library
Also Do validate the validity and failed attempts made by User.
Let us know the details.
Regards,
Ram