drupal problem with virtualmin

virtualmin is running fine.

I created a domain for drupal installation.
Drupal site is installed successfully. But I found that after installation of drupal, all files or folder under /default folder under /sites of drupal root folder cannot be modified , deleted.

I checked the file owner is same as the domain default admin user. Even though the domain admin user ssh to the server, files are still unable to modified, deleted.

system return permission deined.

I found that somehow the file write attribute will be removed from from owner, even I chmod to 755 .
Every time I reload the drupal site, write attribute of file owner of /sites/default/ will be removed i.e. 555