I had some regal lillies off my sister in law some time ago. They were marvellous for 2 or 3 years, then they went pale and sparse flowering. One day I saw a red beetle on the leaves, then another and another. The dreaded lily beetle visiting. Also vine weevil may be attacking the roots too. Especially of potted plants of all kinds. Rather than try to rescue.... might be time to start afresh with new stock and keep a watch and catch them and kill them.