@jay9044 I'm not sure how the Trello/Zapier integration works, but I know of a couple other integrations where Zapier has a deduplicating filter in place, so when you add member xyz to the card it will trigger the zap, then if the member is later removed from the card and then later member xyz is added to the card again (if the deduplicating filter exists) Zapier will not fire since it sees it as a duplicate. -- This is just a possibility, I'm not sure if the Trello integration works like this, but might be a way to explain why your zap hasn't fired multiple times in your testing.
The other possibility is that Zapier only checks for new members every 5-10 minutes etc, so if you add a member and then remove them before the 10 minutes are up when Zapier polls Trello for new members on that card the member has already been removed by you. -- again some assumptions here, just something else to check for in testing.
One way to test the zap is to create a new card within that list and add members to that card to see if your zap does indeed trigger (and if your filter is working correctly this time)