Any charitable group should make it a firm practice to send thank you's promptly. At our church it often seems that only those who are prominent get thank you's and I frankly think it takes away from the generosity of others who are not so involved. You might want to take a little time away from the situation and then write a note telling them about the importance of saying thank you. Don't send it off right away, but let it sit a while. Look at it again to be sure it's not too snarky and adjust it if you must. Or if you do want to be snarky, send them a package of thank you notes and tell them they must have run out! Don't let the rudeness of others muddy your kind heart.