You say that it used to work, but no longer works. Are you able to identify some changes that you’ve made? Presumably, you upgraded this plugin, BuddyPress, and WordPress. Do you recall any details about exactly what changes you’d made around the time it stopped working?
Can you confirm that you’re not seeing the Group Blog option *either* in the main group navigation (accessible to regular group members) *or* in the group administrative section?
Can you share details about how you have BuddyPress and BP Groupblog enabled? BP Groupblog must be network-activated. Do you also have BuddyPress network activated, or is it active only on a certain site? I wonder whether there might be a problem with load order, so that BuddyPress is incorrectly being fully loaded before BP Groupblog has a chance to initialize itself. Here’s a test you might consider running – put the following in your theme’s functions.php:
add_action( 'bp_include', 'bp_groupblog_init' );