ThreatLocker uses the directory in C:\Windows\Temp in order to place files for installation on agent upgrade. This directory is often used for many different software applications, and could potentially be full of other temp files.
Deleting the temp files from this directory and restarting the service should remedy any agents that are failing to upgrade. Follow the steps below:
Note: This affects all version 6.3 and before
Open Command Prompt and navigate to the C:\Windows\Temp directory
Enter 'Del *.tmp' to remove any and all .tmp files in that directory
Navigate to the 'Computers' page and search for the device(s). Click the checkbox to select the device and click the 'Restart Service' radio button located on the top ribbon.
Upon service restart, the agent should be in the process of updating.