irritated

I don't think I've ever mentioned it, but I volunteer for a local humane society as a foster parent for cats until we can find them a good home. I've been doing this for several months now and have had 7-8 cats in and out of my house in that time. This is on top of the 3 of my own I already had, and the 1 foster kitten we recently adopted to make it 4 of our own. Right now I've got 5 cats in my house, meaning 5 cats to feed, clean up after, and give attention to.
Anyways, a couple of my cats have come down with some health issues that we (and our vet) feel may be stress related. So I told the humane society that I can no longer foster for them and need someone to take the foster cat I have right now because she is causing some problems in my home. I got an email back basically saying "we'll see what we can do, but in my experience these health issues are not caused by stress". To which I replied that in our opinion and our vets opinion, the stress of strange cats in the house is the only cause we can find and whether or not they agree we would like to try removing the stress and seeing what happens. Also, that I have had some personal issues lately that are preventing me from having the time and energy to keep up with so many cats. And..... I have not heard a word since. I sent these emails out on Friday. Now, I realize this may not seem like a long time to wait for a response, but let me tell you this - whenever I have emailed with offers to help, I have received a response within hours. Whenever they have asked me for help, I have responded within hours. This is an all-volunteer organization, so I understand that they rely on volunteers to foster, but I also know that they have a 'core' membership that is in charge and could easily take in this cat.
I guess I'm just really peeved that I have done a lot to help them out, yet when I ask for help in return (meaning please take back this foster cat that I am under no obligation to keep for you) so I can take care of the health of my own cats, I get nothing from them. I'm debating what I should do if I continue to not hear anything from them. Threaten to take her to the pound? That's what the people who contact them about strays are threatening when they call me to take in a foster to save it's life. Seems to get a quick response, but I shouldn't have to go to that extreme. I'm at a loss for ideas......