I am running an experiment by recruiting participants from Prolific, and providing them with a link to the study. In order to participate, they have to click the link, enter their Prolific id, and then enter the 'waiting room' until the session begins. 

The trouble is that many participants enter their prolific id, and are admitted to the waiting room. And then, when I create the session, they are told the 'session is full', or they remain stuck at the 'waiting for session to begin' page. And this problem does not show up on the screen where I am monitoring the room, because these individuals never make into the session. 

This has created a number of headaches for me. Some of these people are disappointed they cannot participate, others are angry. 

Has anyone encountered this problem while using rooms?

Are you using oTree HR? That is the recommended way of launching studies on Prolific: https://otree.readthedocs.io/en/latest/mturk.html#prolific

It's not recommended to use the "session-wide demo link" for real studies. That is just for demo purposes and will lead to the error you are witnessing.

Hi Chris! I appreciate the quick response. 
I am a bit confused. I am not using a session wide demo link, but I did use a Room-wide URL. I was under the impression that the best way to deploy the code to a server was to use Otree Hub, and the Heroku serve deployment seen there.

Can I use OTree HR if my project is not built using Otree studio? Is there anything I can read about OTree HR?

yes you can use oTree HR together with oTree Hub / Heroku.
Create an account here: https://otree-hr.herokuapp.com/accounts/login/?next=/