Oops Aqui, not sure. Only thing I would say is don't chuck it yet! Move it maybe! Do you have it as marginal or bog plant?
Also, although I don't seem to need Oz's miner's helmet to catch slugs and snails and puppy dogs tails, would recommend search in darkness with torch to try to discover culprits! It could be that it might be being attacked by something in the pond, which may be why Margaret suggested lobelia is not suitable marginal. However, as I said before, have a feeling that it may be regional