I've had that same issue with 2 users now.
one I never got to actually troubleshoot (time resolved the issue... don't ask, I don't know either)
I tried following up and they told me "it just started working"
no changes were made on my side.
The other is/was getting hung up at "Checking Compliance".
I opened a case and worked with an engineer for 2.5 hours and we couldn't get it to work.
It was an older machine that had previous issues with malware/viruses.
The department head decided he would replace the machine.
No resolution was ever found on either case.
so.. 2 isolated incidents with odd enviornmental issues. Other than that... 99.5% of my users have no issue with Host Checker.