I'd guess there's something wrong with your project. Try a Save As and republish. Be sure you remove all unused Library items. At worst, copy all slide and paste into a new project and set all the publish/scorm settings again.
I've seen this before myself and never found a logical solution, I don't think... Though do check the quiz preferences for things like how many times the question can be answered...and be sure the timeline is correct (nothing stopping the playback before buttons are shown).