Hi Andy, thanks for posting - I just checked the forum and picked up on your problem.
This puzzles me since when I add the line Needs["SpinDynamica`"] to the top of your file and run it, starting from a clean kernel, I do not get the behaviour you describe (see attached file). One possibility is that you were trying out some of the features in the documentation, such as the setting for $Chronology (end of part 1) and that a setting was thereby changed. Mathematica has the feature that anything you do in a session persists for the rest of the session, so maybe that is the problem.
Please retry, starting from a clean kernel (i.e. restart the kernel) and using the Needs line above to load SpinDynamica. Let me know how it goes.
All the best
malcolm