I can't find a reference to this problem, but I know of at least one other person experiencing the same thing.
1. Open a new contact sheet tab
2. Double click an image to bring up the preview window
At this point there will be a checkbox in the lower right of the window, used to tag the file. I have noticed on both windows and Mac that after doing tagging for a while (maybe 10 minutes), it can take up to 5 clicks on that checkbox before PM registers that a click has occurred, and marks the checkbox. Very frustrating, as it means I have to look at the checkbox to make sure when I click, it is actually selected.
Is there a work around for this?
...Mike