Course completion issue

Hi

I have a complex assessment where the learner must fill in multiple fields per 'question'. This is not a problem as I can keep track of scores manually, save off their input into an 'essay' question for saving to our LMS. As all the 'questions' are essay type I've used javaScript to manage the completion status using  SCORM2004_CallSetValue("cmi.completion_status", "completed");
SCORM2004_CallSetValue("cmi.success_status", "passed"); (or the 'incomplete' version as appropriate.

This all seems fine and reports the status and score correctly, BUT when returning to the course from an 'incomplete' status course 'completes' straight away.

Looking at the logs in SCORM Cloud, when the course launches the status is 'incomplete' but if I put a debug javaScript call using SCORM2004_CallGetValue("cmi.completion_status");  as the first trigger on the resume slide this reports the status as 'complete'. SCORM Cloud logs show the status change approx 0.5 seconds after reporting as 'incomplete'.

Anyone come across this or have any ideas? (can't share the file on here unfortunately)