Thank you for taking the time to visit the Community to post your issue, although I apologize that you haven't received a response.
This issue will need troubleshooting to help isolate the cause of his behavior. I would recommend running the Validate & Repair tool on the data file from the File menu > File Operations > Validate & Repair option.
In the Validate File window that opens, click the top box to "validate file" and then click OK.
When the validation completes, a data log will open in Notepad, please let us know if any errors or issues are found/repaired, and when ready close and re-open Quicken.
Specifically, when a known monthly income is received, but not necessarily on the expected date, say 3-4 days later, then the next due date is pushed forward to 2 months later, rather than the next month.