Replication Monitor Missing Subscriptions in Watch List
Update for resolution if someone is having similar situation.
I faced similar scenario where I could not get subscription agent list pulled up when launched replication monitor neither I could get the result from Sp_replmonitorhelpsubscription and the query kept on running without result. When checked currently executing query, all the  distribution agent jobs were being blocked by couple of pull subscription distribution agent also causing issue for other distribution agents jobs and couldnot pull replicated commands. I stopped and restarted those 2 head blockers distribution agent jobs that released the rest of distribution agent jobs. This resolved the issue with subscription agents not being showed up on the replication monitor.