Does this issue occur with a specific QVW? All QVWs? Is issue reproducible on-demand? Have you checked the QVS Event logs for errors/warning regarding this QVW noting the timestamp of when issue occurred?
If there aren't any errors present in the QVS Events log that allow for further research, suggest you open a Support case and provide a sample QVW which exhibits the issue along with the detailed steps needed to reproduce.
Principal Technical Support Engineer with Qlik Support Help users find answers! Don't forget to mark a solution that worked for you!