Hi Simon,
I agree completely with Dan that you should try the SCORM debugging available in Moodle. Something else we do for all SCORM 1.2 packages are to run them through SCORM Cloud before pulling them into Moodle. We check all ways through the course, passing and failing to ensure the scores are both reported and reported accurately (e.g. percentages vs points) to the LMS. SCORM Cloud is free for this type of testing. This way you know if your SCORM package is working on its own or if it's a Moodle settings isssue.
Also, what version of Moodle are you running? There were some updates to the SCORM module a few (or more) versions ago that fixed a few things too.
Finally, what version of Storyline are you using and what settings? We had to choose very specific settings in both Storyline and Moodle to ensure things are reported correctly. Having your results slide before the last slide of your course works well too.
Good luck!