Hi,
Use transaction ST05 and record SQL trace. In this trace check if you see accesses to buffered tables. If there are, maybe it is related to the type of sql statement , e.g. it could be with explicit "bypass buffer" or with options like "order by", "group by" and so on which bypass the table buffers too.
http://help.sap.com/saphelp_nw2004s/helpdata/en/aa/4734a00f1c11d295380000e8353423/content.htm
Best Regards,
Sylvia