It might happen when working with many tasks that you cannot select some of those that are on the bottom because the list refreshes too often.
To solve this problem:
1. Navigate to the program's installation folder.
2. Open up the PQRefreshManager.exe.config file (if you can't find this file install the latest Power Update version).Make sure you have the permissions to read and write the file so after editing you can save the new data (or copy the file to your desktop, make the changes, and copy it back).