There's a temptation to give them a similarly vague answer, like "Then do it correctly next time."
?

I'm suspecting pilot error.

Maybe the problem was that they were using the Sign Up form and didn't understand that it will send them an email message to confirm their address - if the confirmation email goes into their Spam folder, or they just don't see it, and as a result they don't confirm their address that might seem like setting the password "didn't take".

Another possibility is that they typod their own adress and didn't notice. So the confirmation was lost in the mail, and the password setting "didn't take".

Yet another possibility is that their email service greylisted (deferred) the confirmation - thus causing it to take much longer than normal to arrive.

Shal