[ns] I can not create a new object.

I installed ns-allinone-2.1b6 but I had problem with the command ./configure.
Then I installed ns-allinone-2.1b5 and all was OK.
I put also ../ns-allinone-2.1b5/bin into my PATH environment and ../ns-allinone-2.1b5/otcl-1.0a4 into my LD_LIBRARY_PATH environment.
So I ran ./validate and all the tests were OK.
So I created a new tipe of TBF that I called NTBF.
I saved this new file in ntbf.cc and ntbf.h.
I added ntbf.o in the section OBJ_CC of Makefile.in
I modify also the ../ns-default.tcl with the new parameters.
So I ran ./configure and then make from the directory ns-2.1b5 and everything was all right.
I had also the object file ntbf.o in the directory ../ns-2.1b5
But, with surprise, if in my program I call new NTBF I can read :
invalid command name "NTBF"
    while executing
"NTBF create _o34 "
    invoked from within
"catch "$className create $o $args" msg"
    (procedure "new" line 3)
    invoked from within
"new NTBF"
From another computer, with always ns-allinone-2.1b5 and the SAME files and executing the SAME procedures, I had not problem.
But why? Why?
Maybe I have to remove ns-allinone-2.1b6 from my computer?
In the other computer I had also the ns-allinone-2.1b6 and all was OK.