I have split this thread into a separate discussion, because I think it deserves it
After investigating, I found that only students who have a grade for a given HotPot are shown on the reports for that HotPot activity. This is not what one might expect. This applies particularly to students attempting a HotPot for the first time and who do not complete the HotPot (i.e. keep going till they find all the correct answers). These students will not appear on the HotPot reports until they complete the HotPot, as only then will a grade be added to the gradebook.
I am pretty sure this explains why your students were not appearing on your HotPot reports.
I have fixed this issue in the HotPot module for Moodle for 2.x. The fix will be available to you as soon as the HotPot module is updated on your Moodle site.
FYI, I have also fixed the issue of students' grades not being recorded if they do not complete the HotPot. This fix is also contained in the latest version of the HotPot module.