By default, queued reloads will fail after a 45 minute timeout is reached.
Error: Failed to allocate a QlikView Engine
Error: No QlikView Engine was available after waiting for 45 minutes
Note, this setting exists to prevent tasks from running even though the file lock may have been lost, which may result in tasks eventually running but not being able to complete due to the saving of the file not being possible given the file lock has been lost during the wait for the QVB engine to become available. Be cautious in changing this setting.
This is an adjustable value.
You can find this in the QVDistributionService.exe.config file (C:\Program Files\QlikView\Distribution Service):
<!-- The number of minutes to wait for a free Qlikview engine -->
<add key="QvbWaitTimeoutMinutes" value="45"/>
This value may be set as high as 300 minutes from recent testing in the 11.20 and 12.xx tracks, as it appears there have been changes made in the .Net Framework that allow for a larger setting now, but going higher than 300 will likely result in the distribution not working correctly due to not being able to save the distributed copy of the source document, or the source document not being able to save if the task started later than 300 minutes after the initial start request.