If you're using Pool Rules, you can see which participants are blocked by various rules in a few different places. What this boils down to is depending on how you setup Rules, Ethnio will block you from emailing them from Scheduling or Pool and tell you why. So a quick example:
You'll see each email that's blocked shows why they are blocked. In other words, which Rule specifically is preventing them from being emailed again. Another place you might find information on why someone is blocked by rules would be Scheduling >> Calendar:
Notice that you can't Invite or Confirm Manually because this participant is blocked my multiple pool rules. Pro tip: rollover the blocked button to see all the rules in a tooltip.