I don't think homepage blocks are currently implemented. Your best bet will be to make your plugin a generic plugin, and look at the hooks in the template files. (Look e.g. at the Templates::Index::journal hook, specified in templates/index/journal.tpl.)
As for why the plugin is only appearing on certain pages, have a look at http://pkp.sfu.ca/wiki/index.php/Writing_a_Block_Plugin
, specifically the part about registering the plugin with the database.
Public Knowledge Project Team