We are setting up a new moodle site. the old 2.5 site didn't have the following problem.
We have Moodle 2.7.1 and Captivate 5.5.
If force new attempt is yes courses run through fine and reports how many attempts you take, however you can't drop out of a module half way and come back to it later as it starts a new attempt and marks you down as incomplete and failed.
If start new attempt is no you can drop in and out fine, but if you fail and need to resit the course again it over writes the previous attempt, so you are always marked as having 1 attempt.
We have a load of health and saftey courses where people are allowed three attempts to pass and if they fail all three need to go on classroom based training.