Most often this is because they are adding a period onto the end of the password they were sent. 

The default temporary password email is formatted like this:
Per your request: Your account password has been changed to «Password». For security reasons you will be asked to change it when you first log in.
Log in to your account at «Account_Login_URL» to access your forms.

There is a period at the end of the password provided that often confuses people. The period is not part of the temporary password so make sure they are not putting it on the end of the password. They will have to enter the temporary password to login and then again to create a new password so we suggest they copy and paste it to prevent typing it incorrectly. 

To prevent this confusion in future it is recommended to change the formatting of the Password Reset Email so something similar to below:
Per your request: Your account password has been changed to:
«Password»
For security reasons you will be asked to change it when you first log in.
Log in to your account at «Account_Login_URL» to access your forms.

You can do this by following these steps in Gifts Online
Launch > Applications > Forms Manager
In Forms Manager click Settings in the top right then Account Designer
In Account Designer click Email in the top right then select "Reset Password" from the list on the left of the page. 
The email template will be shown in the middle of the screen and can be modified here.