It's hard to know without seeing your workbook, but I can think of one reason.
Tableau is only going to plot one x/y coordinate for a given location. So is Tableau actually plotting both, but they're one on top of the other? I ran into this situation when I was mapping by zip code - regardless of what dimension I had, Tableau plots the center of the zip code, so all my marks were sitting right on top of one another, with only the top one visible. I resolved this by slightly altering the lat/long. So instead of the generated lat/long, I did it along these lines:
IF Cust=CustA then lat + .0000001
ELSE IF Cust=CustB then lat - .0000001
ELSE lat end
IF Cust=CustC then long + .0000001
ELSE IF Cust=CustD then long - .0000001
ELSE long end
This resulted in 4 marks plotted very close together, but not right on top of each other. You may have to experiment a little bit to figure out how many decimal places will work best for your situation.
Many thanks, Cathy.
I think this is what I need to do. Just alter one slightly.
Thanks for taking the time.