Just another quick post to remind players that if Peter is missing from your game or a main Questline part is not triggering, go look for him in your Alternate Quahogs.
This issue has been happening for a while when Peter is in an Alternate Quahog either on a task or just wandering, therefore you need to go there to get him and trigger the Questline part. Peter should then follow you back to your main Quahog.