After recently installing A1 Click Ultra Cleaner I was reasonably happy with the results  While it did not remove all"problems"  it explained that this could be because either they were open files or folders that were not empty.  I generally had less than 1 MB of unresolved issues.

However, after now doing several consecutive cleanings using the "Clean PC "option it detects 548 MB of space being affected involving some 48 problems.  The last run only cleaned 2 of 48 problems.  548 MB is a lot of disk space to be ignored.  I notice these 46 problems all involve Temp files.  For example:

C:\\Windows\System32\DriverStore\Temp\{a8b500e-8743...}

Why is there this huge difference in unresolved disk space and how can I correct the issue?

Follow up to previous post (...huge amount of space uncleaned)

I went into Disk Cleanup and check marked additional boxes and then cleaned up.   I ran another A1 Click scan.  Issue appears to be resolved!

My guess is that the Time Buffer checkbox in Cleaner Settings was checked.  The Time Buffer protects you by not deleting files that are less than 7 days old, but it does mean that not all junk files get deleted.  In time they will be deleted.