[support] new modules don't install properly

I recently had the experience of installing the webform module on a D6.19 site. It showed up on admin/build/modules as enabled and was installed on sites/all/modules, but the config menu never showed up even after clearing the cache and couldn't be reached by typing in the url either.

Now the same thing has happened with the user_import module ie. it enables but the modules menu doesn't become available. The site status report seems fine with no major errors showing up, and the the access log doesn't show any errors.

So it's looking like new modules don't work on this site and I'm at a loss as to what the problem might be. Any ideas?

Marty