Did you check the QDS logs? Is there any connectivity issue between the QMS and QDS? What is the status of the tasks? I would start with that and if you find any error check the community or our KB ( https://qliksupport.force.com/QS_CoveoSearch). Cheers!
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I agree with @Albert_Candelario in that you need to review the QDS logs for the date(s) of the issue(s), particularly the QDS Root log. The QDS Root log will note if there are any changes to the trigger status of a reload task, so you should be able to narrow when a task is skipped and potentially why. By default, the QDS Root log is stored in %ProgramData%\QlikTech\DistrubutionService\1\Log, and has a naming convention as Root_<date>.txt. If you have configured QDS logging to a different location than the default, you can verify the location in QMC > System > Setup > Distribution Services > General Tab > Application Data Folder field.
If you can review this log and provide more information, we can continue to assist you.
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