sendkeys problem for MDI form

In VB6, I initialize APP.Helpfile to the help file XXX.hlp my co-worker created.  I am sure that the XXX.hlp works.  However, when I show a MDI form and it did not show the help file and freeze either press F1 key or click the menu which will execute the sendkeys "{F1}" command.  When I click one of the menu of the MDI form and bring up another form, and the F1 key works either the form is a MDI child form or not.  I am sure that my co-worker VB6 program works and his program is similar to mine and I could not figure out the differences.  Please help!