You can change your password on the login page or in your account area.
This is almost always because of the way you are entering your username or email. Both usernames and email are case sensitive, so James is not seen the same way as james. Please make sure you are entering the correct letter case.
The most common reason for this is because customers unsubscribe from all emails. You can manage your email preferences in your account area on the website.
You can change your email in your account area on the website.
If this is the case, you have logged into two different accounts as your account is shared between the app and website. An easy way to confirm this is head to your account area on the app and look at your email address and then head to your account area on the website and look at your email address. If they do not match, you have created two accounts. This often happens if you have already created an account with username/email and password and then you login with Apple or Facebook and your Apple or Facebook email address does not match your JSA account email address.