Well, it isn't entirely necessary to feed your anemone. Sure it might speed the growth process, but I can tell you that I have 4 anemones (2 BTA, 1 RBTA, and 1 LTA) and I only once in a blue moon feed a couple of the BTAs. Strong lighting, stable/premium water quality, ample flow ar really all they need for the most part.
I will tell you that I believe they are shrinking due to other issues. If you don't have enough light or flow typically they will hunt for it and move all over the tank. A commony overlooked component is a skimmer. Anemones are one of the most delicate creatures and are very sensitive to unstable water conditions. Also the lower the volume the more unstable the water conditions.
I tried to keep 2 BTAs one after the other in my Nano 24 with halide and nice flow and they shrunk from 7" to a dime size in a few months. I did everything in my power to keep the water quality high, but nothing worked. It might have worked much better if I had a skimmer, but later I got a 90gal tank moved them over. Over a few months in the 90 they increased in size to about 5-6" without being directly fed. My LTA has refused to take any food since I got it 3 months ago and it has gone from 4" to 6". Part of it is that I do feed my fish and corals so there is some form of organic material in the water. The other is that I have 1000w of halide lighting on it. But my point is that I rarely directly feed my nems and they do just fine so I think you are right that something isn't right but I don't think it's a feeding issue.
