Need help with my Chattanooga Train set for my boys

[Nick Dee]

Please help me. I went to a train show with my boys.  I bought an HO Chattanooga train set.  I do not have much money so this was a big purchase for us.  My boys are young (7 and 5).  The set worked fine for the most part.  However the tender box will NOT stay attached to the engine.  I inspected both pieces and there does not appear to be any damage or anything missing.  I tried to go to the online diagrams but there is not one for htis model. 

I appears that there is a hook on the tender box that points upwards.  But this "hook" is no more then a centimeter high.  It looks as if this hook is supposed slide upwards into a creavas in the bottom of the engine.  It fits but seems awfully stiff...meaning that there does not appears to be any flexibility.  It works but only for a little while.  When the train takes a turn they become dettached.   

I tried calling theBachmnsusotmer service but no one seems to answer the phone.