Checkmate no longer running

I have just noticed that Checkmate (1.1.8 (3317) has completely stopped working. I am running Catalina (10.15.1) on an old (mid 2012) MacBook Pro with a new SSD (APFS). The check is set to be made every day. Running TechTool Pro (11.0.5) shows an "unable to dismount drive" error, but Disk First Aid shows no problems with the disk.

I am fully backed up to Time Machine, but uneasy at the absence for a regular Checkmate run. What is happening please?