I assume your document is being hosted by QlikView Server, if so you could check the Events log file to see if there have been any problems recorded.
This would usually be located in C:\ProgramData\QlikTech\QlikViewServer ,but you can check your QMC to find the location if its not the default.
This could also happen if that document was reloaded whilst people where using it, in that case you can configured the behaviour so that your users are not interrupted.
I would also check your server RAM usage whilst users are clicking around, to make sure its not a resource related problem.
Last option is to enable the audit logging for this document, you'll then be able to log what each user is clicking on and maybe see if there is a particular problem chart or set of selections.