Okay, I have investigated and the result is that the C4 scene was playing, that was not intended.
This shouldn't stop the C5 scene from also playing, however the C5 scene is one of the last things that can happen during the BBQ and if you leave to do another scene, such as hunting Haily down or going off with Sami, then you will not be in the scene for the C5 event to happen.
None of the scenes available in the loop should EVER permanently lock themselves for any reason (so if that is happening it's a bug). There was a bug a few versions ago that removed Alex's moonstone which might be a cause, but it doesn't seem likely for you? The only actual condition for the scene to happen is to have the conversation with Alex about contraception, which it also seems like you had as it was lighting up in the guide.
More than that? I dunno... That's everything I can think of for this.