Chose different blogthemes, but only show the default theme

Issue Summary

I installed Blogtheme few months ago right after I installed Drupal 6.x. It worked fine, since I did not have much cointributed module at that time. Everything working yokay that users can change their blogthemes and their blog will be refreshed with that blogtheme.
However, after few months' time that I installed so many contributed modules, then I tried to re-visit the blog theme function...it does not change the blog theme anymore even user select a different blogtheme than the default.
Do you have any ideas what could be the possible contributed modules causing the failure? or if there is some other reasons?
Is there any better way to diagnose what are the cause of this failure?
Thanks.