If you do a Google Images Search on each of the names and stamp you will see a stamp for all but two of them. There is one more that is iffy because it isn't just a one person on the stamp but a family. I may eliminate that one but will wait for another clue.
Hmmm... I guess I'll have to search some more since you all only eliminated 2 and I got 6. I'm thinking I goofed, lol!
I googled American Philatelic Society and found the site stamp.org. Then, I searched each name on the stamp.org site to see whether the person had a stamp. According to their search engine, 6 didn't have any results, but perhaps it just didn't find any. I'll have to do some more research.
ETA: Ok, I now found a list of all people featured on stamps on wikipedia and can only eliminate 2, like the rest of you. I feel better now!