I have experienced the same issues... I am about to lose my 2nd anemone. I have 2 small/medium black oscellaris clowns (spelling). These 2 clowns are quite aggressive with the anemone, but what kills it is the robbing of food. The clowns have yet to figure out to bring food to it. Even when I spot feed the anemone, they rip the food away as if it was trash, so the anemone starves in time. My current one was extremely healthy when I bought it. Took about 3 months for them to thrash it to its current state. I currently have separated them, so the clowns have no idea their toy (anemone) is on the other side of the tank trying to heal itself.
It isn't taking in a lot of food, but it has a fighting chance now (fingers crossed).
Best food I have found so far is going to the grocery store... ask the meat guy if they have any mussels or oysters that have just opened. They usually will not sell these and have to throw them out, however they are still very good for cutting up into chunks and freezing. Then just take the chunks and thaw them out as needed. I use shot glasses and store them in the fridge the day before use.