If this is because of complicated javascripting then 
Httrack may not be able to handle it correctly.  You may 
also get problems if the webserver is looking at the 
HTTP_REFERER header and seeing things it doesn't like 
(because HTTrack doesn't access pages in the same order a 
regular surfer-user would do).

You may want to try alternative web copying programs if all 
else fails (but I've usually had the best luck with 
HTTrack).