The ability to resume from where you left off is controlled by the 'Lesson Status' - you can only resume an attempt where your lesson status is currently 'Incomplete'. It's possible , in some circumstances, that the lesson status can be set to 'Failed' rather than incomplete when the user exits the activity thus preventing the learner from resuming/completing their previous attempt.
This situation typically occurs where the scorm package contains a learning element and also a quiz and uses the incomplete/passed lesson status pairing. If the authoring tool used to create the scorm has created a 'Mastery Score' in the imsmanifest.xml file then the learner's score will be compared to the mastery score when they exit the activity. If they have achieved less than the mastery score this will override the lesson status and change it to 'Failed'
Solution - when creating the scorm package, the option to include mastery score should be set to no. If the authoring tool does not have the option to exclude a mastery score, then the imsmanifest.xml should be edited afterwards to remove the line containing the mastery score. Note that the mastery score must be removed completely - setting the mastery score to 0 will not have the desired effect as exiting the course early would instead give a passed rather than failed status.