Your question is a little confusing, you say you bought it for your grandaughter and you also say your grandaughter's one works fine? Assuming we are talking about two different machines and yours is brand new out of the box, is it possible to take your machine to your grandaughter to see if she can get it to work? Maybe you are doing something wrong or have assembled it incorrectly or maybe there is a certain technique that involves pushing and turning the pitcher? - She may be able to identify the issue, even better if she's a nice grandaughter, she'll come to you and help!
From my quick research on your behalf, it seems a lot of people have had the same problem and it was down to alignment of the safety tang you initially refer to.
Good luck and go easy on the eggnog :-)