I'm not sure why you would be experiencing this and I don't have an easy suggestion of a setting you could change that would immediately fix it.
For a problem like this, I suggest contacting our support folks at:
By email: support@camerabits.com
By phone: +1 503.547.2888
Hours: Mon-Fri, 9:00am-5:00pm [PST/PDT]
I'd recommend just giving us a call, but if you aren't able to do that, or want to get started by email, you can send us your log file. You can find the log file by going the Help menu in Photo Mechanic and choosing "Reveal Support Data". This will bring you to a file called PM_log.zip, which you can send to us by email.
